I think this question was addressed recently in Writing Novels, but don't quote me.
Honestly? I don't think we'll ever come to a consensus on this issue, because there are two schools of thought on SYW crits: Tool for the Writer and Tool for the Reader. I think there is room for both.
I rarely post my work for critique, but if the SS that y'all did such a fine job workshopping last fall is ever accepted for publication, I don't think it would ever occur to me to delete the draft.
Why? First of all, SYW is password protected, so there is no chance of the draft showing up in a Google search later on down the road. Second, there were some wonderful points made about certain things, and I feel that every crit in SYW is a potential learning tool for another writer. Sometimes I browse through old threads, and if I notice that the original post is gone, I won't continue to read. The critter may have made some great observations that I could use as a learning tool, but nothing is left to reference.
I don't see posting a few chapters of a novel as being any different. It's not the finished piece; it's not what will eventually end up in an editor's mailbox or a reader's hands. No one is going to point and snicker and say "Ooh, Joe X is a terrible writer, look at his first draft." Readers don't care about the first draft, they care about the finished product, and whether or not it's worth the money they paid.
If someone doesn't want their work archived in SYW, use private betas. Or perhaps the writer asking for a crit should state up front that they will be removing the source material once they have gotten what they need out of the thread.
I'm not asking for change in SYW, and I doubt we'll ever come to the point where a rule such as "Thou shalt not delete original topics" will be enforced. That seems silly. There are hundreds of threads in SYW, and I'd wager less than ten percent are ever deleted. It happens, it just doesn't feel like an epidemic.
That said, I love SYW and its role here at AW. I feel that SYW is about more than just what the thread participants (both OP and critters) offer to each other. It's also about what the threads offer to our community as a whole.
Drat, on that note, I am almost late for work! *races out the door*