Government Executive has a piece on firing federal employees. It's all very nice, but it misses an issue which can be as important: the economists call it "opportunity costs".
A manager has many demands on her time, demands mostly over which she has no control. It's the in-basket, which keeps filling up. In an office with several or many employees, there's also an urge to devote time to your employees, and to be fair to them. (Not that I achieved that, but I could be made to feel guilty about failing.) And if you'd like to think of yourself as an effective manager, you probably have dreams of your own you want to implement. (I had too many.)
Now if you have an employee who's marginal, what the rules say is you need to devote time to him: counseling, training, documenting actions, etc. etc. The rules are all very well, certainly they fit the golden rule, they're what I'd want applied to me. But spending the time is the killer; it takes away from the in-basket, it takes away from paying attention to other employees, and it kills your dreams.