Semis can be used one handed, which can be a great benefit in situations where your arms are otherwise occupied. If you have ever tried to fire accurately with a pump while trying to prop yourself up from a prone position on your elbows, you know what I mean. Semis can be a crutch, however. They really have no place in normal rec paintball. This makes it so that I can't switch it to automatic mode on the field.
Automatics have their place in scenerio games.
In these games automatics are the heavy artillary on the field. They are very good at cover fire. Provided you have a player with deep pockets. I hope that answers your questions. I prefer using a pump, although I have found I enjoy my TS-1 very much because of its profile.