Mirror image is a very, very low dps increase, and it won't affect Force of Nature (which we still don't know what it will do). Unless Mirror image mechanics change radically between cata and MoP (could happen I guess), I'm not impressed.
I suppose your right in saying that, but Mirror images can be very useful for us as it very irritating how the FoN currently works, you can lose a lot of DPS by using the CD that increases DPS? The way the FoN mechanics are at the moment are, in my opinion " terrible", due to you losing a cast just to pop your FoN CD, but combined with Mirror Images, the time taken to cast them, and the DPS you gain form it, may or may not be worth it.
the ability is designed to give you more utility than giving you flat DPS boosts. So which move is best for you may depend on the encounter or the opponent you are facing in PvP
If taken from its current state, Chains of Ice would be extremely overpowered on a Balance Druid right now. You have a class that can shapeshift off all snares/roots, can move faster than any other class (outside of sprinted classes), and now you give them a spammable 60% slow. Melee would stand no chance
Granted this is assuming everything stays the same (which I know it wont). Point is if your expecting to get Vampiric Touch or Avenging Wrath you are most likely going to be disappointed. Otherwise it will help give better utility and survivability in both PVE and PVP for the cost of nothing and creates a fun engaging choice. With the possibility for marginal DPS/HPS increases.