"The only reason that place is so popular is because everybody goes there."
From the "I don't care to belong to a club that accepts people like me as members" department, it's interesting to note that the hottest tickets in technologically-enabled "personal networking", Orkut and GMail, are invitation only.
"Evil" or not, it's brilliand marketing. The rationale for Orkut could ring true; after all, the highest-quality networking contacts are made by drawing-in to the network, rather than by reaching-in.
The GMail manouver is clearly a pure marketing ploy, though the real aim is obscured. Google understand the importance of image ("don't be evil") perhaps better than any major player in technology. Their feel-good image has bought them fierce loyalty from many of their followers; challenging the integrity of Google (even with sound arguments) is a sure way to get onesself roundly excoriated. At least until the privacy and legal issues can be worked out, it's as beneficial to Google to to have GMail available only on a limited basis as it would be to have it available to all.
Belay that: It's actually better, since current legal challenges are such that Google's ability to advertise on GMail is, at present, severely constrained.