I think concrit is important if you have a real want to get better at the craft of writing, whether it be for fandom or otherwise. I welcome concrit myself, but I'm also familiar with people giving me their opinions, because I frequent writing groups and things.

My big problem with fandom concrit is that people will think they want it, and ask for it, and then get defensive when you offer it. Or counter with my favorite: I wrote this for ME and not for ANYONE ELSE. To which my response is generally '...er, why did you post it then?' Of course, these days I only give concrit to people I know well, only when it is solicited, and I always temper it with things I liked about the piece as well.