Personally, I am seeing several inaccuracies amongst not only the OP here, but also some replies in opposition to him.
There are some problems with the cleric class, however most of them (in my opinion) are not in the class design, but with the player of those clerics. This is not to mean that the players are bad, rather their intention or attitudes don't work well for the cleric class. To further specify, the only comments I am really making here are in regards to the healing clerics, not the battle clerics, as they have plenty of threads of their own already.
A healing cleric can easily be the most active and busy member of a group, even if never entering into a battle themself. Playing a cleric can be very costly and very frustrating...it is not meant for everyone. Playing the healing cleric takes a certain type of person, and again not everyone finds that type of play fun, and apparently more people play clerics than people who find that style of play to be fun.
To be more specific, in regards to the OP, I do not think cure spells should be mana-free. Just think where the clerics would be if DDO went even closer to the D&D rules and only allowed you a certain number of spells from each level before having to rest instead of having mana which lets you cast many time more spells. Clerics also gain other benefits because of their lower mana count, which although usually ignored or taken for granted is partly why they get lower spell or mana counts. As some have mentioned, going mana-free isnt going to happen. All MMOs need money sinks and clerics tend to take the largest chunk of being that, but it's generally a false sense of need rather than the actual option to do so that it is.
As for those stating that a cleric should rarely ever need to resort to alot of wand or scroll usage, that is somewhat true and somewhat false. It really depends on your playstyle and attitude towards playing a cleric. It is true that you shouldnt need to spend much on such items when the cleric is exceptionally picky about how or who they play with (of which many do this, and it greatly lessens the clerics available to many groups, which is an adding factor to the bad name of clerics everywhere). It is true, that those clerics who only group with guildmates or specific friends will generally go through much less supplies than any other cleric, and by limiting oneself to such groups, is easy to claim that a cleric should not need such supplies or funding for those supplies. The false part comes in, because not every cleric is experienced, nor does every cleric decide that they will only play with certain people or groups (thankfully they all do not do this or a large partion of the game would find themselves with no available clerics ever).
As for the idea that clerics cannot cast offensive spells and be healers at the same time, while preserving mana, that too is false. They most certainly can, and in many situations, the use of such offensive spells can save alot of mana from not having to heal people as much. Knowing which mob to cast slay living on, or when to comet fall an oncomming group of mobs can save a ton of mana by making the kills easy on the group and requiring little to no healing being cast during the fight. Clerics have quite a few nice offensive spells that are very beneficial to the group as well as very beneficial to the cleric's mana. The determining factor to this, however, is knowing when to use them and when not to.
To be the most beneficial healing cleric to a group, many things must be combined and acted upon accordingly within any given group, and the group itself determines the values each thing has in importance. Again for those who only do guild groups, they do not have to give much thought to things as they know exactly how their friends play and they know what to watch for, and can negate many portions of trying to determine what holds the most importance. This also greatly helps in negating the necessity for lots of extra supplies in wands and scrolls. However, we all know that such groups are not going to happen, and there are many, many PUGs that will negate all such rules and expectations. Sadly though, many clerics will simply avoid those groups, and by doing so, it is very easy to say that there is no need to spend alot of money on items or you're simply playing poorly. There's a reason why such clerics wont join PUGs, and thats largely because they know very well that in doing so it will involve spending alot more on items to keep the group alive.
All in all though, to be the best healing cleric you can be relies on experience and quest klnowledge, and ifnally adding in an understanding of your group's make up (be it friends or strangers). All 3 things must be taken into account, however their individual importance is based upon your playstyle. Some clerics prefer to conserve mana and use wands and scrolls instead, while others reserve wands and scrolls as a last resort. Both styles work quite well, however one will be more costly than the other. Neither is wrong, just undertsand how you play and what your needs are. Try playing both sides, you might find the other more beneficial to you. Nothing will make playing the cleric easier than simple experience, as you begin to learn what is most effective where, when the group needs to be at full health and when it doesn't, etc. KNowing the quests is also very important, as you know what buffs are needed and which ones aren't, when you need to conserve mana and when you can dump it quickly, etc. Group makeup is also important. If you know your group and hwo each person plays it greatly lightens the load on the cleric and they know who to watch and take care of first. In a PUG, the place of the cleric becomes much more hectic, as you do not now how each person is going to play, who is going to be the one to run off, who is going to accidentally pull the next group of mobs before you are ready, etc. All of which equates to more mana getting spent unnecessarily. However this will happen in PUGs on a normal basis. Typically, PUGs tend to be the big reason so many clerics go into hiding. It is more costly in mana and supplies, and many PUGs are less likely to offer donations to cover these supplies to the cleric, instead leaving the cleric to foot the entire bill to help allow the group to succeed, which in itself is not entirely fair, but it happens alot. This also adds to the bad name of clerics, even though in these cases it isnt the cleric's fault, instead they become money conscious over mana conscious. We all know most quests will not produce enough in loot to cover the cost of even 2 cure serious wands, and many a cleric can find themselves broke if people don't help out or they dont have other characters to send money from.
All in all, the cleric is a great class, but probably the most abused. They are fully capable as they are now, if you are able to play it well, and again, not everyone can or wants to play that way...clerics are not for everyone. Most other classes are rather forgiving in their playstyles, whereas clerics do not get nearly as much room in their playstyles if they are to be a healing cleric.
On a final note, no...clerics are not required in every quest. In actuallity, clerics are not really required much at all, except for a few certain quests where people maintain a common method of finishing a quest (although alternatives are there, just that most dont use the alternative methods). Many classes can heal, and in good groups these other classes can fulfill all healing needs just fine. The problem comes in when people playing these other classes simply refuse to do the healing when needed. I've seen plenty of rangers, paladins and UMD rogues, who either dont know they could even heal or use wands, but who simply refused to even do it. In a good group a couple of these would be all that is needed, and that's not even taking into account the beneficial nature of having a good bard along.