There are more (and stronger) benefits to staying pure than there are to multiclassing, in my opinion. However, I too have been sorely tempted to multiclass, so I know why it is done. Sometimes you want to play one way, fight one way, but you want benefits that another class can add. For example, I am a barbarian, I deal significant damage (especially when raging), have decent saves (particularly against traps), and some damage reduction. That's in addition to AC from armor (always light). I also have increased movement speed.
I see no drawbacks to being a barbarian... except low spot, low search, and the inability to disable traps or open locked doors.
What about the typical barbarian's complete lack of self-healing? Pots only go so far at cap...
But do I multiclass to gain those abilities? No. Why? Because I don't need to. It would be easier to, yes, but there are other ways to gain the same effect.
How can I circumvent the low spot and search?
WHy would a barb care?Well, when I was a wizard, I saw that there is a spell of Detect Secret Doors. If there's a spell, then there's a wand or a scroll. My understanding of scrolls is that, assuming your UMD is high enough, you can only use it once. My understanding of wands is that they have a specific number of uses, called charges, and some regen while others don't
Only eternal wands recharge, of which there are only a select few, and very limited in capability. I actually saw a wand of Detect Secret Doors... so if I really want a rogue's spot/search abilities, I can just redo that quest and nab it... and invest in some UMD next time I level.
Barb rage interferes with UMD, making the skill almost useless. There are ways to make it work, but its not worth the character investment.
How can I circumvent the inability to open locked doors?
Party with a rogue. I assume there's a wand or scroll out there for the Knock spell,
Which doesn't have a DC high enough to hit most anything after a few levels. though I personally haven't seen one, and with a decent UMD I could use such an item. There are also Bells of Opening in the DDO store, each one comes with only a single use but a single bell only costs 67 TP. Which isn't bad at all. So to open locked doors, I don't need to multiclass either, I just need to find an item with Knock or buy a Bell.
How can I circumvent the inability to disable traps?
Run around the trap? Most can be avoided. Otherwise, bring a trappy capable toon. Well, there is no easy answer to this, but personally I don't think trap disabling abilities by themselves justify the need to multiclass. If you really want the traps to be disabled, instead of relying on your ability to go around or get through them safely, then you can always add a rogue to your party. I wouldn't suggest multiclassing as a rogue to do this, but that is also an option.
Any spell that exists probably has a wand and a scroll that casts the spell.
Only up to level 7 are widely available. So instead of multiclassing into a spellcaster, spend points into UMD and keep your eyes open for the relevent items. For instance, wands of healing are especially useful.
Now, there are some class specific abilities that you won't be able to emulate without a level or two in that class, such as a bard's songs, a barbarian's rage, etc, but why would you want a barbarian's rage if you're a ranger (for example)?
There are other reasons to multiclass barb, for instance, than rage.
If you are determined to multiclass, there are some things to be aware of. First of these are the stats. If you want to multiclass a barbarian, then you need to use another class that focuses on strength and constitution, such as a fighter. If you want to multiclass as a rogue, you can do so through the dexterity focus or the intelligence focus
Fail, such as a ranger, dexterity based fighter, or wizard. There are a lot more combinations that would work, but I won't go into detail on those at this time.
Rogue levels are splashed for: trap capabilities, UMD, EVASION, EVASION again, and sneak attack DPS. Barb's taking rogue gain evasion, pallies gain evasion and UMD, etc.
Before multiclassing, or even before creating a class for the first time, there is one bit of advice I always give. Do your own research into that class. When choosing a race, keep an eye on what benefits it gives you through enhancements, and what classes correspond to those benefits. Human seems to give average benefits, racial toughness and an enhancement bonus to any
two of the six stats. So human is the only race that is easily useable no matter what your class.
Some races work better with certain classes, but with enough experience you can make any combination work. You just have to do the research and be prepared for trial and error. That goes double for each additional class you multi into.