To be honest, I think you might be right in that he hasn't cost us many points with his errors but that is more by luck than judgement. There have been too many errors that luckily for us have gone unpunished and that shouldn't go unnoticed just because of luck. That also isn't to say he's done a 'bad' job. He has done better than I imagined a League One second choice keeper would have done stepping into a team unexpectedly.What "costly" mistakes has Wilson made? As in directly cost us points? Not saying he hasn't but I've not attributed much blame his way during his stint in nets.
On a separate note, this isn't a slate of Kelly at all as I like him a lot, but it concerns me that everyone has decided that he was the entire reason we won games. It is like saying Wilson is better than Marosi because we have had more wins since. There has been a distinct more defensive system change that coincided with Wilson replacing Marosi and around the period Kelly came back in. I think people shouldn't look at a teams win % with a specific player in and treat is as gospel. Not saying you have by the way.