Slower adoption? By default a site like this is going to have no traction, since there are a gazillion other reddit clones around. But make it appear exclusive, and you immediately have people begging for invites.
(Edit: I don't mean to imply that hype-generation was the real reason to make it invite-only. I'm sure that the stated reasons are the real ones. The interest generated by exclusivity is just a nice byproduct.)
However people are hearing about the site to even care about getting an invite, or know what it is, those are the same circumstances that they could happen upon the site and want to register. I don't think I've ever heard of a principle that stated invite only leads to greater adoption for a new site than open register, it's rather done to retard growth so the new infrastructure can keep up, or in this case to control spam. I believe slow growth, in absence of traction, is a perfectly valid way for many sites to grow and eventually gain traction.
(Edit: I don't mean to imply that hype-generation was the real reason to make it invite-only. I'm sure that the stated reasons are the real ones. The interest generated by exclusivity is just a nice byproduct.)