If you want strong HoT HPS on a single target you're going to have to pile on some talents. With CW, Cultivation, and Germination, you can do some impressive single target HPS. Without those, not so much.
But if you're raiding, it's mainly the AoE throughput that counts. Rejuvenation doesn't heal for much per second, but you can keep casting it on more and more people. It all adds up, and in the end somewhere between 2/3 and 3/4 of our healing in a raid should be from healing over time spells (artifact traits included). Considering that most of those HoTs double dip on haste, it's a lovely stat.
Probably not. People were already using Cultivation for progression before the buff. Cenarion Ward is bound to see more use, but that's just ~25% uptime on one target and doesn't make much difference. Also note that Cultivation scales with haste. So while it does add another HoT to trigger mastery, it also benefits from haste (and crit, obviously).