From the 5.4 PTR patch notes:
Seal of Insight no longer has a chance to restore mana on attacks.
I made an art out of smacking bosses to preserve part of my mana pool.
Magmaw? Yeah I’ma stand in melee and smack the boss, ty.
Feng? Right click that sucker.
Now they want to take that away from me? With a fixed mana pool, we need *more* ways to get mana back, not less. Sigh.