That's certainly an interesting idea - mostly everybody should know someone who has a gmail account, so if you get a couple invites a month, that should be plenty and the setup would

Well I was about to say destroy scammers, but I just realized that they would send out spam to places where you could gamble your invites for Real Cash(TM) or just straight up buy them.

This would lower the creation of accounts, but then they would be rarer and worth more to spammers, since a spamming gmail would be rare.

And we would hear sob stories of people getting their accounts closed for inviting spammers.