I understand and somewhat agree with your point, but sometimes people anticipate that, given the incentives of the job, they're unlikely to be any better than anyone else when push comes to shove (your pt 2). Think of "golden handcuffs" as one example.
This particular example doesn't apply to me, but I think I'd personally be able to rationalize a lot of selfish behavior following that logic so I try to avoid putting myself in situations where it would come up, even if both points might apply.
That part on its own is not enough. Only when: 1. someone else will do <the job> AND 2. it would be better for you to be doing <the job> than someone who takes it up then you should take it instead of them.