How many is plenty? Unless I've been surrounded by pure gebius my whole time in I can't recall a single sacking for trade work...I've seen one or two assigned to bay duties because they were more 'suited' to that kind of steady work...
Personally I have dealt with about a dozen over an 18 month period, whilst in AIR casework. As KG suggests,and also what I hinted at as well there are invariably other issues as well that are in the mix with these cases.
A sad reality though is that many of these inefficient people just get transferred to other sections as the chain of command either can't be bothered to deal with issue appropriately or haven't got the moral courage to do something. In my experience the worst offenders seem to be the techie world with my own trade not to far behind.