I would also guess the invitation method is also aimed to stem the flow of crap by making sure it's actual humans with a valid interest in Pinterest vs pure spam.
By monitoring the behavior of the person making the invites you can possibly assume the invited guests will also be spammers and toss those invites when you boot a troublemaker.
Making it by invitation also creates some interest and mystique.
I am pleased that webmaster World has added Pineretest to its Social Media Forum.
I remember a year ago everyone was saying that Facebook does not work etc etc. Yet their number of members keeps going up, and their advertising revenue is now billions.