Premise 1: The ability to cast spells in armor is not very powerful.
(Justification: Psions can do it, and it isn't an issue with them. When you come down to it, it's a few extra points to your AC for a class which very well might not care about AC anyway. IMX, casters generally don't bother with armor anyway even if you houserule out ASF altogether.)
Premise 2: However, not ever caster should be able to cast in armor right off the bat.
(Justification: Wizards generally wear robes, and your average Bard just shouldn't be tromping around in full plate).
Premise 3: The characters that Arcane Spell Failure screws the most are gishes, who are substandard in power level anyway.
(Justification: They want to wear armor and need the extra points of AC to do their job, but doing so means spending feats to reduce the ASF, and since they're supporting both melee and magic, they need double the feats of a normal person anyway).
Conclusion: If Arcane Spell Failure were a consequence of nonproficiency, gish builds would be capable of wearing armor but it would still be onerous for straight casters to wear it (they would have to either multiclass with fightery base classes or blow feats, both of which are significant investments which show them making effort to learn how to do this). This is also a more elegant solution than having to specify that a given class (like the bard) can cast in a certain weight class of armor when that's the weight class he's given proficiency in anyway and then patching the problem for everyone else by charging extra feats on top of proficiency.
*bump*
Sorry, but I think this is important. Getting around Arcane Spell Failure is just not worth spending the amount of effort 3.5 and the Pathfinder Beta require.
