I don't get the set bonus.

First, regrowth is basically used as a flash heal and/or to have the chance to use swiftmend at any time. The 6 extra seconds only help with the swiftmend thingy, and not so much since you'll probably use it before the 21 seconds a normal regrowth lasts.

Second, lifeblooms are not supposed to explode while in raids. There are certain situations where this helps, but since the final heal generates aggro on the target it's not a good idea to spam it on a regular basis.

Tier 4 bonus is way better.