The Restoration druids in particular are unhappy about this change, especially when taken in context with the recent change to the game that will allow everyone to see what magical buffs you have placed upon you. What this means is that just about every time a druid casts Innervate -- the spell that would benefit most from the change to Spirit -- there is a high probability it will be immediate dispelled. Now we have heard tell that Spirit as a stat will be changed somewhere down the line, that it will begin affecting spell damage as well as mana regen, so perhaps this is the first step toward that change. It's tough to know for certain.
What we do know, however, is that the change makes the PvP gear more attractive for use in PvE, where Spirit does help with the longer battles. But I have to wonder, exactly why would the developers, who have professed that they want to create two separate gear sets for PvP and PvE, make the Season 3 Arena sets more PvE viable? It may be that they want more people to participate in Arena battles, or it could be for some unknown reason I'm not seeing. Help me out here. Do you think the change from MP5 to Spirit is a buff or a nerf?
Update: Another patch just pushed through on the PTR, and this change to the Druid healing S3 set has been reverted. The gear will retain its MP5 stat bonus.