Its a by-product of the aggro model. Focus on one target for healing. The basic encounter is is your DPS faster in overcoming the Boss' HP reserve than the Boss' DPS ability to drain or outpace your healers.

To go to a multi-tank solution encounters would need to be wholly redesigned as to how aggro works. Or, in the interim, you'd have to see more encounters like Patchwerk.

How many tanks can you have on one boss? Or how many tank targets can you have before a fight is about that instead of any other mechanics?

Yeah, I just don't think 5 tanks in a 25-man would work.

I always had issues with the labels OT and MT.

First, some fights Blizz sets up don't have well defined MT and OT roles like Razorscale. Both tanks need to get adds and swap on the boss.

Second during a raid, I switch tanking depending what the tank is good at. For example as a DK with inpterupts, I get Jax and the warrior/paly with stuns can control the adds.

Third, it really implies a ranked position between the two as if MTing should be reserved for the better tank. In some fights, I want the more geared or more experienced tank on the adds. There are plenty of fights were the "OT" position is a lot harder than the "MT" position.