DPS is awesome. You have 15 DPSers on Patchwerk. They kill him in 180 seconds. Their DPS doubles after a few months and you kill him in 90 seconds. Hooray, progress! Your raid can move faster, there's less time for a boss to gib someone randomly, dangerous abilities like Polarity Shift happen less total times during a fight, so the randomness and the chance of failure decreases significantly.
But not everyone is so lucky. Healers see it the worst. In the beginning, Patchwerk does, say, 30,000 DPS to your tanks. Each healer can sustain 5,000 HP/S, so for a good buffer you have 7-8 healers. But your tanks' gear increases. Incoming damage drops 20% across the board, and you're taking 24,000 total incoming DPS. At the same time, healers' HP/S goes up 20%, so each one is healing 6,000 HP/S. Now it only takes 5 healers to reach a solid buffer zone. The same thing can happen with tanks; In the beginning, you need multiple offtanks to help with adds or some such, and as time goes on you can drop a tank out. That's a somewhat rarer situation though.
This is an extremely annoying effect of gear increases. I mean, it's possible that you have a few people who start out enjoying healing but later want to switch to DPS, or your healers don't care about stepping on each others toes and like to be able to slack off. Who knows. I consider it a problem, though, that as gear increases, healer numbers can dwindle. The other issue is massive healing variance per fight. For example, patchwerk takes a solid amount of healing. However, many fights can be done, and done more efficiently and faster, with less healers. It's not currently feasible for healers to switch to DPS on the fly since the healer requirement changes too often and costs too much. However, with the advent of dual-specs I'm betting we'll see a lot of guilds pick 2-3 healers as swaps, who'll switch to DPS relatively consistently. Blizzard has declared repeatedly that they won't redesign fights under the assumption that you can just ask half your DPS to dual-spec to tank or anything, but I wonder if we'll see much tighter DPS constraints relevant to healing considering that dual-specs have made it easier to minmax.
I wish I had some sort of solution to this issue, but I really don't. I can't think of a way to make it so that healers and DPS see efficiency increases without eventually decreasing the healing and tanking requirements. That said, I feel healing as a whole needs a revamp. More later.