The problem is that the people who are doing it to become a professor are playing a game that is stacked against them. And if/when you lose that game you're well behind your peers in opportunity cost (see my other post).
I know several brilliant scientists (postdocs and PhD students) from my time at Princeton who had / are having a very hard time finding an academic job. At some point even the purest motivations will succumb to feeling used in these circumstances.
In math it is sort of hard to outsource the boring parts of your project to people working beneath you, so I didn't hear people complain that they were getting used. But I can imagine that this might not be the same in other disciplines.
I know several brilliant scientists (postdocs and PhD students) from my time at Princeton who had / are having a very hard time finding an academic job. At some point even the purest motivations will succumb to feeling used in these circumstances.