If they accurately estimate the amount of time that they must spend to get the job done to the specified quality (get some specified GPA, for example), and then put forth that effort, then they have demonstrated some combination of intelligence and conscientiousness. It doesn't matter to me if they are smart enough to succeed while partying every night or smart and conscientious enough to study three nights a week and pass, or conscientious enough to study five nights a week and pass, because their performance is adequate in every case.

Lots of jobs are perceived to require attendance, even in the white collar realm. For those jobs, a college degree is less effective signalling than a work history showing a long employment at a job that requires attendance.