Assuming characters in your raid are randomly distributed among the 30 specs, and with two out of 30 specs able to provide the debuff, you'll have a 50% chance to get it in a 10-man raid and an 82% chance to get it in a 25-man. Of course, Shadow priests and Balance druids aren't really as prevalent as other specs in most raids, especially Balance druids, but we have no idea which specs will be flavor-of-the-month in Naxxramas 2, so we'll just leave it at that.
If you're going to a 10-man, then, you might assume that there's a 50% chance that 3% spell hit is going to be provided for you, one way or the other. Is this enough of a probability to be willing to gear for less spell hit? In my opinion, it is not. And sure, you could maintain multiple sets of gear, for varying levels of spell hit provided by your party, but that just gets to be a pain.
I have a few suggested fixes for this issue.
- Make Improved Faerie Fire part of the base Faerie Fire spell. That way, it's available to all Druids, and your chance to get it raises to 76%/97% for 10/25-man raids respectively, which is probably enough to gear for.
- Give both Improved Faerie Fire and Misery +melee and ranged hit as well. IFF actually used to have this, but it was removed. As far as I know there are currently (in beta) no melee hit chance increasing debuffs. This would make Misery and IFF useful even if all your casters are already at cap; melee DPSers can (almost) always use more hit.
Either one of these would solve the problem, in my opinion, and I don't think adding melee hit would be overpowered. If they baselined +spell hit on Faerie Fire, they could then change the IFF talent back to adding melee and ranged hit, so Balance druids would still bring something special to the raid.