The downside is this:

If you cast a HoT with this style of haste, it may run out too early. The first tick or so may get them to full health, making the next tick wasted. A longer wait between ticks could lead to a person getting injured and needing more healing.

Given, you would need to be healing in a way where you end up overhealing, but I have used this tactic in the past.

One example is when I'm healing Hodir, during Frozen Blows (or whatever it's called, where everyone takes a ticking damage).

I'm at the gear level where I just need to Rejuv everyone once and they'll be back to full health or near full at the end of Frozen Blows. But say my haste causes my hot to tick faster, then at the end of Rejuv, the Frozen Blow has not yet ended, but my Rejuv has expired on most of them, and the next tick of Frozen Blow will damage everyone.
Also, because it ticks unnecessarily faster, I'm sure a large amount of them are wasted (over healed).

But I agree, when I'm healing MT (5 man or 10 man), faster tick is always nice and I'd never complain. This is why I vote for glyph to allow haste to affect only selected spells. For example, I'd glyph for Rejuvenation to NOT be affected by haste, but for Regrowth (including the hot part) or even Lifebloom to be affected.

Hendrata: well you can also look at it like this. On Loatheb (outdated boss, but anyway) you got a small window for healing. I tended to apply Rejuvenation on as many people as possible, and get a few ticks on everyone when the window appear. With haste I can squeeze in more ticks, and thus more healing, for the same amount of mana.

That was going to be my comment too.

"you will be able to do more damage or healing... but then again, you'll be spending more mana to do it."

Isn't that what haste already does for all other spells? Haste makes fireballs or whatever cast faster, so you can cast more of them, and do more damage, but the amount of mana you spend doing so increases. I don't see why extending this concept to Over Time spells is so hard to understand.

As a tank, I can say the more potential overhealing the better. Bosses hit hard, and healers already have to be chain casting their heals b/f the hits even occur. Being able to squeeze more hot ticks in b/t successive 20k boss hits could mean the difference b/t the mt dying or not. Mana isn't an issue for Druids atm, so this is a straight up buff for them at least.