I think they are good thing to have, but as for their prominence they seem way on outskirts of the site.
How important (visited, useful) they are to the site? How much effort should be spend on writing and maintaining them?
I would like to find some time and start working on wikis (there are next to none so far), but I have doubts if they matter much and if my time might be better spent on other things on site.
Edit I am talking about full tag wiki pages, not small excerpts that are used as tag descriptions.