Also keep in mind that when the fan does fail your readings will be less accurate than a passive shield. Though as I remember one member here who considers a FARS almost an absolute necessity didn't seem to notice for some time when one of his FARS failed. Now there's discussion on another thread about how the New Improved Temp/Hum sensor may need to be replaced up to a couple of times a year when used in humid environments with FARS. Even without FARS the replacement rate appears not to be acceptable, as I understand it, but FARS is suspected to greatly aggravate the problem.
So FARS will increase cost and maintenance and, depending on your situation and view, either marginally or greatly increase the accuracy of your station, assuming you you replace the FARS components before they fail and the sensor when it starts to drift out of spec, otherwise you may be worse off with the FARS.
You may want a FARS if you see the need to have highly accurate readings for important scientific studies that you're doing as a professional meteorologist or something. If you just want to know what the weather is like in your backyard, you may not need one.
Personally, I see siting as far more important, as CW2274 has said. I think probably relatively few home weather stations are properly sited, and I think it would be better to stress the need for that over FARS.