[Afton]

I don't think this comment deserves the downvotes it got even though I disagree.

Consider your example: If you pay a tradesperson to 'do a job' you should expect that they will cut every corner available to them that isn't in the contract. If you are paying for time, you might expect them to do a more thorough job. Which is better depends on a number of factors.

[jfk13]

The risk of paying for time is that the worker may deliberately (or even subconsciously) take longer than necessary to get things done.