If I'm still getting qualified, interested traffic from a Wikipedia link, why would I even care about the
PR impact of their "no follow" rule?
Let's not lose sight of the big picture: Google PageRank and, more generally, search engine optimization, are just means to an end: traffic on your site that converts to whatever business objective you have. If a link drives traffic commensurate with the cost/effort of getting it, proceed! If not, rethink your plan ...
On a technical note, Google's Matt Cutts has been pretty definitive that the "no follow" tag does more than just prevent a search engine from "following" a link. It essentially negates a link from being counted as a vote (hence preventing the passing of any PageRank value):
Matt says on his
site,
"The nofollow tag allows a site to add a link that abstains from being an editorial vote. Using nofollow is a safe way to buy links, because it’s a machine-readable way to specify that a link doesn’t have to be counted as a vote by a search engine."
Although put in the context of someone buying links, his advice translates directly to Wikipedia: their use of the tag means you get traffic, but not PageRank.