I've spent thousands and thousands of hours on tractors. Every single one of them have been without a cab. For some jobs, having a cab can be a pain if you need to get on and off the tractor frequently. But that is about the only downside I can see with one.
We just bought a new tractor, and again we didn't get the cab. It wasn't that we didn't want one, it was that the extra money a cab would cost would prevent us from getting the size tractor and implements we needed.
If you can afford a cab, then there is really no reason not to get one.