Tim Mayer once told me that less than 2% of users used any advanced search features. So, it'd be a great feature for a very few users. Is it worth it? Maybe, if it generates buzz with the tech sites.
DuckDuckGo has done a great job of differentiating in part by marketing stuff most people don't care much about.
I'm a bit surprised it hasn't cost them the Google ad syndication partnership by now given how many studies they've done flaming Google.
Maybe it's just the way I search, but, Wikipedia aside, I rarely see any results from the interior pages of wikis, knowledge bases or directories. (Okay I recognize that most directories left standing are crap.) It seems to me that the good ones are nodes of expertise. I do see a lot of specialized forums in the G and B serps.
Through directly tracking over 1 billion end users daily, Google can see what sites users are actively looking for, how well people engage on them, and which sites people repeatedly choose to visit.
A forum is highly interactive. People actively seek them out to learn, to answer questions & to participate in some sense of community. Whereas a static directory typically doesn't add enough value & isn't differentiated enough to be a sought after destination which people seek out as an alternative to a search engine. A lot of wikis are overrun by spam or so adverse to the risk of spam that they close themselves off to edits from new users & thus die slowly each time an old user quits for whatever reason.
If you edit / write / contribute there is some sense of ownership & identity wrapped in the content consumption. If you are exclusively a reader the commitment is often much less unless you are reading things vital to work or things that confirm your identity & political outlook & such.
