Ridley's Cohort said:
No offense intended, but I think you should consider your own question carefully before answering.
Stop and look at other 8th level buffs spells. They are pretty freaking powerful. We see things like Mind Blank, Protection from Spells, and Iron Body. All of those look a lot more impressive than 5-12 points of Dex.
In the PHB there were only physical buffing spells. However, the addition of splatbooks has given us the wonderful Cha, Int, and Wis boosting spells. If you are spellcaster a 5-12 point increase in your spellcasting stat for hours is an incredible boost.
Also, I found 8th level abjurations to be severely lacking. Mind Blank looks good on paper but it is really on good for PCs with crappy Will saves and sorcerers and wizards are not. Protection from Spells is not a big boost in saves if you have already maximized your resistance bonuses (e.g. +5 cloak). Also it is costly to cast repeatedly.
Iron Body is a joke. What kind of arcane caster would cast this spell? You would have to cast all of your spells Stilled or face a huge spell failure chance. Also 50/+3 DR isn't tremendously useful against the powerful foes you'll be facing at Level 15.
The real problem with x3 empowered Cha/Int/Wis buffing spells is when they are used in conjuction with other splat books. Spell Power, in particular, combined with the x3 empowered scenarios can lead to utterly obscene spell DCs.