This question concerns the reasons why prescriptivism is criticized. I know of these criticisms because I hold them myself. However, I have a question about the opposite side, and that is, do any professional linguists hold a strong prescriptive belief, and criticize descriptivism as heavily as the other side criticizes prescriptivism? If so, where can I find information about them?
Only people on the border of linguistics would hold (read: propone) the belief that a language should be spoken differently than the common usage of native speakers, as far as I know. Linguistics is a science (which involves measurement), and those who say the observed facts should not be what they are do not show themselves to be scientists. However, some linguists publicly recognize the value of the notion of standard dialects for participation in the majority (or dominant) culture.
(My apologies for the author's terminology -- it is a bit dated and offensive to modern readers)
I can't imagine there's a modern professional linguist that would think this way. In fact, I don't see how you could get through a modern undergraduate linguistics major, let alone a master's or doctoral program, thinking this way.