Why? Because you loose a whole category of spells, access to magical items and such, while you only gain a few spells in your "mastered" school as well as some pretty dull bonuses.
A regular wizard with the right feat progression and some money that "specialize" in a school will be much, much better and more powerful than a specialst wizard at the same level, because of that category of spells the specialist wizard looses.
Honestly I feel that if you loose a whole 10 levels of a school, I think you should get a school equally powerful as the one(s) you gave up, which naturally means you know all spells from a specific school if and only if you are powerful enough to cast spells from that level.
Eg. Evoker gives up Conjuration. He looese access to the conjuration school, and he automatically knows all the spells from the evocation school, like a cleric. He casts this spells normally (that is no 9th level spells at 4th level. 2nd level spells at 4th level max). This would TRULY make wizards "magical", as you know all spells of a specific school, which means you have researched some yourself and learned others from other wizards. And no, that's not 100s of spells, I hardly doubt you actually use all those spells avaible from your school. And if you do, good for your wizard for his imagination
I have always disliked the "learning" wizard, that annoying bastard that only read the books and not learn things himself. And the researching rules are too complicated and annoying. This will give the right feel of a wizard, a smart wizard that creates his own spells, and "learn" only a few, if you get my point.
If that's balanced or not... I have no idea.
