Apart from the cost of running the MOT station which I'm sure you're well aware of. You haven't had the extra pleasure of the arrogant VOSA guys who keep the stations in check (They are a breed of special arrogance and SS comes to mind)and love handing out points for any wrong doings that they find. The latest I was told today that you get awarded 500 points if they find you've left your smart card in the reader. Now 500 points is a lot. "You must treat it as a bank card" They say. If you want more hassle in your life go ahead and get a testing station.
By the way they told me today that up to now 3 stations in our area have turned around and said take the lot away we can't be arsed to test any more.