Actually, yes-- if people believed "convertible" meant "physically capable of having the top removed". You can see the flaw in that definition of "convertible".
That's not wrong, but its rightness hinges on a point of semantics. Fighters can use Melt Wood Staves, same as every other class. When the player clicks a button and Melf's Acid Arrow pops out on the screen, has the character just cast a spell or not? Melf's Acid Arrow is certainly a spell. We have a naming convention that distinguishes between "casting a spell" and "activating an item", but when the result is a thing that looks like a spell, feels like a spell, and has the name of a spell, it becomes clear that the naming convention is more than a little arbitrary.That makes no sense -- the ddo logic, whether we like it or not, is at least being applied consistently -- if a class casts spells it is a casting class. If a class does NOT cast spells it is not a casting class.
That makes sense to me. But I figure many classes should be able to "fake it"-- they couldn't form their own incantation from scratch (the spell isn't on their list), but they should be able to craft some stronger magic by using the staff's power as a starter. A paladin using a staff with a spell from the cleric list, for example, shouldn't be quite as good at it as a cleric, but he should be better than, say, a wizard (I'm pretty sure arcane and divine magic are still treated as radically different areas of study in 3.5). I'm not optimistic that we'll get anything so subtle, but we can hope.