Well based on the blue post ...
Taken this into account ... (Outside of the 4pc WG interaction) unless the druid is full health, dreamer is only going to get applied to others in the group when YG heals them. If the druid is full health, then dreamer will probably go on the lowest health target. So a fight where the druid is constantly damage will see very little use from the 2 pc, as dreamer will never get applied to anyone else (except when 4 pc used).There will be some logic attached to the Restoration Druid 4-piece set bonus for it to prioritize anyone except the Druid while Ysera's Gift is healing much more frequently.
After casting WG, then this new behavior kicks in, and YG will heal the lowest health target over the next 5 seconds (5 ticks) even if the druid is not full health. Still unknown though if YG will just spam heal the lowest health target, instead of prioritizing targets without Dreamer. In the latter you would get 5 dreamer hots out per WG, instead of potentially just 1 getting spam refreshed on the lowest target.
So yeah ... the strength of this set bonus really depends on how Blizzard programs it. It could be very strong or very weak.