1) Sustain is a "cast it and forget it" type ability. It's half the cast time of Physick and doesn't require you to target her. The fairy will still take some incidental damage, whether from aggro (a poorly-timed auto-cast Embrace right as an add spawns can do this, easily).

2) Miasma II is excellent for PvP. I'm assuming you're talking about PvE encounters, though, where it is slightly less useful but not useless by any means. There aren't many instances where you'll need to cast it, just keep in mind that it is one extra dot that stacks with all of your other dots, and it's instant, and it hits infinite targets. It's also got a Heavy on it, which is worthy in itself.

3) The unconfirmed rumor is that they're going to fix Obey so it doesn't require an enemy targeted, but I haven't seen any official response on this. We can only hope.