It would totally not be useful. When people are searching they are actively trying to find the answer to a question. They don't want to take extra time to comment. Consider:

1) Google's Experimental Search: http://www.google.com/experimental/a840e102.html. They tried making it the default behavior and no one used it.

2) Facebook: you already have a captive audience to ask for advice on non-timely questions.

3) Delicious: people already save and comment on their search results (the parts of the results they care enough about to keep around).

4) Springpad, Evernote, etc: bookmarklets & cross-platform apps let people save links, web pages, and just about anything else.

5) Instapaper, etc: save content for later.