I think walkons are good to have in general. When I get them into my group in a scenerio, I brief them as best I can, and then off we go. Some of them are fine; some of them are very green, fly off, and you never hear from them again. However, I expect some of them to be like that, just like probably happened in real life from time to time, and I figure they help at least in providing another plane for the enemy to have to focus on.
Also, I think it helps as a way for some new guys to check out scenarios and for old hands to participate in scenarios where they can't commit but would love to play if they are available that day.
The biggest problem with organizing walkons was, in my opinion, greatly reduced in Operation Husky by closing at locking the doors at "start time" and having takeoff at least 15 minutes after "start time". Gone was the process of having walkons showing up all the way to takeoff, which really strained coordination.
This isn't to say there isn't the infrequent problem (like what Skyfoxx is talking about), but I think overall it's good.