I don't know which companies pay attention to their own surveys, but it makes me feel better to somehow make them aware if they have an issue that they need to correct. If I don't tell them, who will? And how will I sleep at night with that on my conscience?
Seriously, if one person hounds them ad nauseam, and nobody else says a word, they may just think the one person is a disagreeable idiot.
Also, if everything was fabulous, I think it's good to tell them that, too. We like it when clients do that, after all.