On the down-side, this all feels a bit dirty somehow. Hard to explain really but it's like a scam to drum up popularity. It's not like a lie but why should we need to advertise (besides those selfish reasons above)?
I don't really see the dirty aspect, myself. If we were putting that statement on non-Perl pages, then I would consider it dirty. But all we're really doing is improving the accuracy of their poor method of tracking language statistics. Yes, it's dumb that we need to do this kind of thing, but not dirty.
Somewhat separately, what's wrong with advertising? I like Perl. I like using it, and I like when other people use it. The bigger and healthier the Perl community is, the more useful Perl is. As long as we're being honest, and not misleading people, there's nothing wrong with advertising in my mind.
I don't feel I need any justification to encourage people to use a programming language that I believe is incredibly useful. If someone hadn't encouraged me to play with Perl many years ago, I may have ended up stuck using a less awesome language.