Because if you have a piece you are really not sure about, and need feedback on, I'd say the chances are that the comments will reflect your lack of belief. You cannot be thin skinned when this happens. If, on the other hand you are certain that a piece you've written is faultless and brilliant and going to knock everyone out, you are inviting the jealous and the not so nice to tell you loudly that it isn't. If you disagree you will just look arrogant for thinking everyone else is wrong. Thirdly if it really is a good piece of writing then it will probably be nicked, and not just by one, but many. I don't think you can win, doing it, unless you are a real novice, genuinely wanting to get advice on the very basics. I'd say the best place for purely constructive advice is an agent, a producer or one of these reading agencies that sends you a critique. Also, more instinctively, I just feel that to ask other punters in such a fiercely competetive business what they think of another competitor's work is akin to throwing yourself to a pack of wolves. There will be some friendly ones out there but also some very unfriendly, unhelpful ones who want to knock your work even if they think it is good. That sort of rank bad(faith) advice will just be very confusing, and probably unsettling. So I just wouldn't do it. The comment I myself made, for instance (though I only made it because the poster has long gone), I wouldn't like someone to make it of me, if I genuinely believed I was right - and the poster in this case did generally seem to think that. My rule on this is take advice from pros but not from amateurs. I don't mean that to sound demeaning to non pros (of which I am one) in any way, I simply believe it is a very sound mantra.
