The thing about it is that Google can route your site - by standard, google wants you to have a roadmap somehow - so it's a good thing. However, it's a bad thing because spammers use it to game google's search engine - to attract more link clicks, ahem - backlinks. To the owner that owns the site, it's a bad thing, because it's a useless account... And it attracts more garbage.
It's a neither nor situation, I think. I disallow Google to index it, both for privacy issues and to stop my member profiles to pollute search results for my site. I want to focus on my actual content, not my members profiles. Also, it prevents any effect silent spam would have.
For years, I allowed this figuring it couldn't hurt. With the increase in spamming of profiles, it's simply not worth the risk. I'm aggressive in tracking down spammed accounts and deleting them. If a spammer visits your site and spams a profile, there is a chance he'll send crap links to your site to build up the link value. By removing access to Google bot in robots.txt, I tend to think it limits my exposure to bad neighborhood linking done by these spammers.