If u think bigger heals means less healing in a raid then ur wrong. When u have more than 1 person healing a tank bigger heals are wasted and become over heals. Its not like ur going to top off the tank and magically hell stop taking damage. You cast a heal and the no big suprise u cast another one. Mp5 is way more important.
The only situation where I can say where mp5 is more important is for raid healers in SSC/TK/early BT/early Hyjal and that's just because raid damage is fairly minimal, but fairly frequent (Naj'entus aside). When you start to develop into later BT - partially Teron and more importantly Bloodboil, you'll need to switch the focus as a raid healer to more Healing/Haste. Mp5 just falls behind as you progress, because fights don't really get that much longer.