A bad employee can affect the whole department quickly - its so easy for corrosive attitudes to spread. But a good employee? It takes time and effort to build goodwill and teamwork.
It is better to remove a bad employee, than hire a better one. More urgent anyway.
> so easy for corrosive attitudes to spread
citation needed
Lifetime of experience. If you have other observations, feel free to make them.
One person's experience is basically useless, statistically speaking.
How about one person, experiencing the same thing dozens of times?