I think a lot of the blame goes into the character designer. I personally think it is good that you can't have an expert fireball slinger and an expert swordsman all wrapped up into one. (Without Gestalt) The fact that multiclassing hinders some character concepts in my mind is a good thing.
But say a character wants to only cast beneficial spells and be a good swordsman. Then they don't care about things like DCs (and to some extent) spell level because typically the only thing effected is duration. And while a longer duration is always nice, scaling duration and scaling damage are not quite of the same necessity.
If a person wants to multiclass, they need to think about it. I think there should be limits on effectiveness. Take the Beguiler or the Duskblade for example. Even though they do get full spellcasting progression of a sort ... their spells are inherently limited. The warmage is a good example of an evocation limited class. While they are still effective within their class, the added effects of armor and such means they're less versatile in their spell selection.
That's really what multiclassing does. The difference (and this is significant) is that rather than taking away schools of spells, it takes away levels of spells.
Me, I like that. But then again I love games where it is RP 90% and combat 10% (or even 95/5). When you take combat out of the picture, multiclassing is a much better (more legit, I suppose) option. An X 5/Caster 5 can be just as good in a minimal combat oriented campaign as a caster 10.
But ... to pull off a combat-minimal campaign you have to have good, dedicated Roleplayers. My kind of folk!