Mana drains as percentage makes int bad

There's been some talk recently about changing mana drains to percentages so that low mana pool specs such as ret pallies, feral druids don't get drained in next to no time in PvP. Nice sounding idea, however this would be a terrible move for high mana specs. In PvP I already value int lower than stam, resil, sp power, crit, haste, hit, spell penetration, and even now spirit. It's a pretty average stat. But changing to % of mana pool drained would actually make it a bad stat to have. Why?

My lifetap will return a static amount of mana depending on how much spirit I have. Drinking, potions, mana gems return fixed amounts of mana too. Say a priest drained 5% of max mana per mana burn. If I gain 1000 mana, it now takes exactly the same amount of time for a priest to oom me. But it takes me more time to refresh my mana pool to full. Thus increasing my mana pool hasn't helped me gain mana, but has helped my enemy to drain me of mana faster.

Possibly the best answer would be to have drains still work on fixed amounts of mana so gaining more is a good thing, but cap the percentage they can take off to help the low mana pool specs?