With your proposed idea, it should be quite possible for a group of: 2-3x shaman + 8-7x hunter to go into a 10man raid and they would all be guaranteed that everything that drops would be usable for everyone in the raid XD And don't dismiss this as impossible since Blizz has even confirmed that hunter pet tanking will still be alive and well come Cata, so it is actually a feasible raid comp with 2-3 hunters using Tenacity pets (and with player/pet scaling changing to 100% their HP should skyrocket.) I would imagine that 7-8 Misdirects on the tanking pets should enable them to hold agro pretty darn well. ^^
It's because of scenarios like this that Blizz will most likely never implement your suggestion.
if they make loot only drop for classes present then people will exploit the hell out of it
just bring one of each class and u can have every armor type covered
holy pally- SP plate
dk and warrior one tanking one dpsing -tank and dps plate
hunter - agi mail
shaman -sp mail
blance druid - sp leather
rogue - agi leather
holy/disc preist - healing cloth
mage and lock -dps cloth
though this does only give u one tank! you could swap the rogue out for a bear druid
basically if u bring a variety of classes then except for items that have people already have very little will go to waste...especially with duel spec and alot of hybrids collecting a second set of gear