From what I understand haste increases the number of ticks you get in your HoTs, but they've avoiding breakpoints by making your last tick include anything unused towards another tick. So if you didn't have enough haste to get a full last tick, you'll at least get a partial from it at the very end of the duration. If that makes any sense the way I explained it.... Though I've heard that WG isn't going by that standard and still has a breakpoint? Not entirely sure on that one
Get more ticks with more haste you just won't have to aim for set numbers as you gain a partial tick of the last HoT, only exception currently being Wild Growth which still does have a breakpoint of 900 buffed I think, whether thats a bug or not remains to be seen
Not really, you are not getting more healing - relatively- at the end. You are getting more healing for the entire duration, since the ticks come more often. And you don't lose healing at the end if there's not enough time left for a full tick.
Also: Tranquility does not scale with haste, and WG still has breakpoints.