I agree with Tasha.
It would be a shame for you to go, Neko. You're a wonderful writer and I would love to read more of your stuff. More, I think this site serves many really useful purposes for anyone who enjoys writing and wants to improve their skills (which is the unending task).
As for the general idea that publishing material here could be detrimental, I have to say that I disagree that it could be detrimental. Yes, if you publish something on here you may be restricted from publishing it elsewhere. Many magazines or contests will require that work be unpublished anywhere. That does not, however, mean that it is going to hurt your chances of getting published with other work.
As Tasha said, you are getting feedback instantly and making fans. Who doesn't love that sense of "they like me, they really like me!"? And instant feedback is nothing to be sneezed at, that is for sure.
A friend of mine who got me into the idea of taking writing a little more seriously had, as his goal, being a fiction writer (he is also a grad student like me and is going to be published first for academic work, something I envy!). He had gone to workshops, retreats, read books. Guess what he was involved in? A writing group. It was an exchange of a group of people who were all interested in writing. They got together on a schedule, which was not all that often, and exchanged stories, talked about what they could do to improve, etc. Well, think of Protag as a free (because Nickb rocks) and monstrous writing group.
For me, Protagonize has become an awesome support group and idea pool. I can read what other people do, see the ideas they have, and share mine. I have encountered some amazing writers here, many of whom either are already published authors, or who will be one day. And I hope that they will, at some point, share that they are published so I can be supportive and get a copy! And I hope they will do the same for me.
Protagonize is a place to experiment, for me. If I have something I want to publish I probably won't work on it on this site, for the simple reason that I might not be able to submit it. But if I want to work on something, this is definitely the place. It is a chance to explore ideas in a place where I can get feedback, get ideas, or even have someone take things in a direction I may or may not have expected! I can work on writing in 1st person present where I have people who will point out if I goofed up, but will be kind.
Without someone to show your work to, aside from a publisher/editor, it is very hard to improve, or to know what your best talents are. Submitted work to magazines or publishers may come back with no comments. You may simply receive a letter saying they are unable to publish your work at this time. You may have no idea why, or what you could improve. This is a safe place to try things out, see if something works or not. You can test something out. If it works, then great, you can use it in a story that you want to publish somewhere. If not, then you know.
More, your presence on this site could be something you can send around. Artists have sketchbooks, and this can be that equivalent. It's a place to enjoy writing, doodle around with words, and learn. You can link your stories to anyone and they can read them, see what they think of your work.
And... I am all for things I can get for free. I love musicians who are willing to give at least some access to their music for free. I love tv for free. I love broadband videos. This is free literature. And so much of it is so good! That being said, if a book ever came out of the "Best of" I would buy it in a heartbeat.