Basically, the m-word and a certain line of spells SCH eventually wound up using better than us. Not wanting to be a support monkey was a fair desire, even if that wound up being its best purpose for the time. Would also posit that the increased flexibility would've done some good for those left out WHMs, but such is the past.
Merits are a bad way to go about such for a few reasons:If a thf gave everyone +5% haste and drgs gave everyone +5% acc, man, thf and drg might've been hugely popular for those buffs alone.
1) Only applicable to endgame.
2) A person that does not have them unlocked and mastered yet would be shunned for those who do.
3) Further shoehorns selection than present.
Don't get me wrong, SE needs to get the stick out of their butt about the ability to max all merits, or convert all abilities/spells to level 75 gains and modified from there, but if we did nothing else to the system and got your idea, it'd just be that much more cookie-cutter.
Essentially, key functions should not be inhibited in such ways.