The concern with windfarms is that they absorb radar energy, creating a shadow within which the radar signature of any plane is reduced/removed.
But you might say so do all reflective surfaces? True, the problem comes as said above, the turning of the blades means the windfarm turbines break through the MTI supression (after all if windfarms didn't show up then neither would a hovering helicopter?). These primary returns present clutter on the screen that can hide the return of a real aircraft; so ATC have to limit any radar service being provided to an aircraft in their vacinity (because you cannot tell if a conflicting track is about to pop out from the clutter)... a wise controller might give this clutter a wide berth, or put another way ATC delays!
There is circuitry that will remove these returns (e.g. RAG Maps as mentioned) but this is done at the degradation of the radar picture (which may or may not be a localised effect?) and again the service should be limited i.e. NOT as good as it MIGHT be!
So, within the world in which we live, ask yourself this question. If you were given the responsibility of signing off an application for another windfarm close to your airfield, in the knowledge that this might mean you have to reduce the service you are providing to aircraft, whose companies might sue if there is an accident (not to mention the hundreds of bereaved families of those onboard)... do you; or would you raise an objection against the planning proposal (which is logged as "we didn't want it, but someone else forced it upon us")...
What do you think?