I know many people who do unit testing (myself included), some of them consultants. Not a single one of them does it "to buff up the invoice". In fact, every single one of them will argue (sincerely) that good unit testing practices help reduce the time they need to deliver code and increase the quality of that code. They'd also argue that good unit testing practices make it easier to then hand over the code to someone else.

This is only valid if the code is not going to be maintained. If it is, and especially if someone other than the original developers are going to maintain it, then unit tests are worth their wait in gold.