The +1 spell per level may be quite useful at low levels, but as the intelligence rises, this benefit is almost completely erased. I would recommend using some other speciality rules or give them the aforementioned +1 spell-power bonus.
I am actually playing an abjurer wizard (in one campaign in which I'm playing) now, barred from the enchantment school, and I think it's perfectly alright, even though we don't have any house rules on this matter. Besides, being a specialist should open for some quite interesting PrCs that normal wizards cannot take.