I didn't have an issue with it because you could (and still can) email inbox@google.com and automatically get one.
If a software company wants to test a new product with a subsection of its users, freely available invitations are the fairest way because the users most interested in the product seek out the invitations. The alternative is to rollout by IP address or some other system that shuts out willing participants.
Perhaps a better system would be more like Blizzard's, which has you opt-in to all betas in your profile, then does a lottery among those users, but Blizzard uses its betas as marketing differently than Google and the products have completely different business models.
Why not have the invitation be going to inbox.google.com? It's already gated behind a different URL; why force additional hoops? To weed out people who would put in the effort to go to the URL but not send an email?
Inbox probably requires Google to index your emails in a different way, for storing extra metadata for snooze returns, tags etc. Hence, there is probably a processing/migration step for would be beta users before they can start using inbox. Hence, just using a different website wouldn't have worked.
If a software company wants to test a new product with a subsection of its users, freely available invitations are the fairest way because the users most interested in the product seek out the invitations. The alternative is to rollout by IP address or some other system that shuts out willing participants.
Perhaps a better system would be more like Blizzard's, which has you opt-in to all betas in your profile, then does a lottery among those users, but Blizzard uses its betas as marketing differently than Google and the products have completely different business models.