Clearly there is a need to capture certain metrics and display that as quantitative information, and the difference between, say, the top 10 and the 90-100 is probably relevant, but the difference within the top ~20 is so minimal its largely irrelevant.
It really galls me when people make decisions about undergraduate or graduate schools (or, probably employees, although this is a position I'm yet to be in) based on these kinds of rankings. Clearly it should factor to some extent (i.e. Harvard vs. Jeremey's University of Rubishville[1]) but there are MANY much more important factors to consider.
The irony is that the rankings are 50% opinion based, which basically amounts to identifying where people think are prestigious. I wonder what people use to identify which the prestigious universities are - they can't possible go and visit them all, but if only there were some kind of online ranking they could use to get an idea...
[1] When I went to undergraduate, this was the fictional university my younger brother suggested he might end up going to.