This is true, but it also dilutes the usefulness. If I tell someone "if you improve documentation, you will get a $10k raise", then I am tying a very well defi

This is true, but it also dilutes the usefulness. If I tell someone "if you improve documentation, you will get a $10k raise", then I am tying a very well defined reward a very poorly defined task. "Improve documentation" could mean fixing some typos. Or it could mean documenting every un-documented public class and method. Or it could mean refactoring code to make it easier to use so less documentation is needed. These all have very different value. In a year's time, I'd have to justify why only one of them actually might warrant the 10k raise. It is *much* worse to tell someone "you were promised a raise... now you aren't getting it", than to never offer or give the raise in the first place.