Applying to a school 1) costs money 2) requires you to modify your statement of purpose. I decided to apply to as many schools as possible (which turned out to be 14). I spent about 2 hours looking at each of their websites and the research interests of the professors, then I spent 3-5 hours looking into the publications of the professors with whom my interests coincided and I tailored a paragraph of my statement of purpose accordingly.
Clearly this much research is not enough to decide on a school. But would you really want to spend hours comparing schools in your mind, to which you may or may not be accepted? For me, I simply can not consider a school properly until I am actually admitted.
The offer that I will be accepting was from a school that I wouldn't have chosen to apply to if I were to apply to only 5 schools. So I am very glad that I chose to apply to so many schools. I think this investment of time is tiny in the grand scheme of things.