A link to the end game spriests doing either of what you said would be more convincing. Many of them have DI in reserve though, so they have a goal of about 400 less haste.
As for our burst, we'd have 2 situations where it's useful in PVE:
1) In the regular rotation, just that you want MB/SW: D to hit harder. Mastery benefits MB more than crit as long as you're proccing orbs.
2) Medium health adds you can MSp/MB spam to kill. Mastery benefits both with orbs, but you'd need SW: P proccing on a main target for orbs to pop up. However, there's other classes with burst that are usually assigned to fast nukes on adds (Arcane Mages, destro locks, etc.), whereas we don't have burst on command.
Overall, mastery is more useful in our best situations. The nerf to multidotting has not changed that we still multidot, and buffing ES helps that. Mastery also benefits MB with orbs, which we do anyway for the ES buff, so we get some nicer numbers from that. Crit is RNG; it increases the chances of a crit, but you're never guaranteed one when you want it (MB with 3 orbs and archangel? Only gonna happen when you want it if you were flying for Alysrazor).
Haste isn't just about those plateaus though. Ignore that we could get more ticks per duration for a second. It makes it tick faster. Everything you cast, but the same damage faster. Shorter MB casts mean more MF, faster SW: P ticks mean more chances at orbs, and shorter MF channels mean more chances at orbs. Haste makes mastery a better stat. It's usefulness is never limited to a plateau for another tick.
Chances are the priests you're looking at that stopped stacking haste are at a point where their sim says to up another stat. It's debatable whether or not you should do that after the second plateau since the sim scaling go wonky for haste around them. Haste isn't worth as much as mastery around that time, but it is if you stacked >300 haste, because after that, haste starts ranking higher again. It becomes a question of whether or not you force your way past that mark and go haste, or do you go with sim and stack mastery (where haste is not likely to pass it again). One thing to keep in mind is that mastery scales badly in comparison to haste. It's a point where theorycrafting isn't going to be able to give you a straight answer.