About tank stance uptime vs healer dps gain, generally it's agreed that the extra healer dps gain doesn't outweigh the tank's dps loss from staying in tank stance for a long period (unless maybe war using unchained), assuming sufficiently competent tanks and healers. There are cases where it's better to stay in tank stance, like a8s 2nd phase with those robots, a tank staying in full dps stance would put too much strain on the healers. However, as everyone probably knows already, this is talking about playing at a high level where people push themselves to gain even a few seconds shorter clear time, and it's generally not a good idea to assume or expect this level of play with PUGs. In JP data center, PUG strats for savage/ex contents often rely on simplicity and ease of execution with little concern over dps/speed optimization. For easier contents like roulette dungeons and normal mode raids, you can definitely have very little tank stance uptime without straining your healers though, especially for war/drk due to their dps contribution (mobs die faster) and self sustain, unless the healer is undergeared or just not very good (I usually do one big pull to gauge the healer's ability to keep up, since for the first pull all my cds will be up generally it's not a problem, unless it's a very big pull like sohr khai before first boss).
As for people following the meta set by the top players, despite playing nowhere near that level, I personally believe that it's because it's very easy to blame something other than your competence (or lack of) when you can't clear something. It's very easy saying that your group is failing dps check because you don't have drg/nin/mch, compared to admitting that some people in your group are underperforming relative to other players with the same jobs (including tanks not going into dps stance enough and healers not contributing enough dps -- I won't argue whether they "should" do that or whether those are their jobs, but in most cases they probably "can" do that to help the group if they're failing dps checks). It can't really be helped though, mathematically there will always be a composition which is better than any other composition for a given fight, unless the jobs are homogenized so much. Though I do hope that the devs will bring the "non-optimal" compositions closer in performance to the optimal composition, which will lessen the emphasis people put on having optimal composition.