I would argue that the current state of the various tanks and their strengths mimics the variety of strengths that the various healers offer. DKs are especially good at magic damage tanking and also do well at AOE threat. Druids are best for large amounts of mixed damage types, a runner-up on high magic damage bosses, and excel when exploring unknown content where massive health is the only answer until you know better. Paladins are especially good at fights that require high mitigation or maximum possible AOE threat. Warriors are good middle-of-the-road tanks for most situations and also excel at mobility and snap threat.

25-man Naxx with 3 warriors? 3 pallies? 3 druids? 3 DKs? Doable... but not necessarily the easiest. A mix of tank types makes things more manageable if you know which to use for which wing and which boss, with a respectable amount of overlap. Ulduar will likely cause us to re-evaluate those tanking strengths and leverage them where possible to help increase chance of success. Healing should be the same, and it sounds like it will be.

A lot of the homogenization of tanks centered around the fact that their abilities were too far removed from each other for each tank to insure some chance of success in all encounters. Druids and warriors before they had AOE threat-generating abilities as of patch 3.02 were going to be woefully inadequate in fights that required holding aggro on multiple targets. We'd have ended up with paladins being chosen almost exclusively as both MTs and OTs in Naxx, otherwise.

Conversely the point as it relates to healing is that holy paladins in their current form are a little TOO different from the other healers, and this is what's becoming a problem. Especially with the upcoming changes. Holy Shock and Beacon of Light will not be enough to alleviate the monotony or the singular role they'd be forced to play as MT + 1 OT healer and cleanse-spammer.