For this reason, rankings (unfortunately) have more value that OP suggests. The "best" students will cluster around the "best" institutions. Does the average student differ muchst between #1 and #5? Probably not much. But the difference between 10 and 100 does, by a lot, and the difference between "unranked LAC" and "top 50 CS" is difficult to overstate.
If you're measuring is "how good is my peer group", rankings are often a bit of a self-fulfilling prophecy. Not so much at fine granularity, but more so if you measure in terms of "20-50 position overlap equivalence classes". This is particularly true in fields like CS, where there are huge differences in curriculum between the top schools and the not-so-top schools.