Computer Science does not get an undergraduate ranking. Computer engineering does--not the same. CS does get rankings as a PhD program, last time in 2014. I've posted on rankings in 2005, on the failed NRC rankings of 2010, on using metrics for rankings, and Bill had his own so-called non-controversial thoughts on rankings.
In the September CACM, Moshe Vardi wrote his editor's column entitled Academic Rankings Considered Harmful
Academic rankings, in general, provide highly misleading ways to inform academic decision making by individuals. An academic program or unit is a highly complex entity with numerous attributes. An academic decision is typically a multi-objective optimization problem, in which the objective function is highly personal. A unidimensional ranking provides a seductively easy objective function to optimize. Yet such decision making ignores the complex interplay between individual preferences and programs' unique patterns of strengths and weaknesses. Decision making by ranking is decision making by lazy minds, I believe.No potential grad student should decide based solely on rankings but neither can we expect them to solve a highly-complex multi-objective highly-personal optimization problem over all 266 PhD-granting CS departments. They will find ways to narrow down their list of schools somehow and a reasonable independent ranking of CS departments can certainly help.
More importantly rankings cause us to compete against each other. Every CS department wants to raise their rankings (or stay on top) and use that goal to work on strengthening their departments and use rankings to make the case to upper administration and alumni to get the resources needed to continue to grow. By the nature of rankings, not everyone can rise up but we all get better in the process.