Amen to that. I've told my kids that their parents have jobs: I am the breadwinner, while mom manages the household. It is therefore their job to attend school. I get performance reviews at work (while mom is of course, exempt from such :D). It is therefore important for them to do well at school, and grades, like it or not, reflect that progress.
Seems to me that the financial incentive has two functions: while it can clearly be a motivator (that likely promotes only short-term and shallow learning; but that's another discussion), it is also an indicator of how parents value (or de-value) education.