Note: this service will be blocked from any company that I am involved with or have influence at.
Why? I'll have a bunch of spammy, disinterested non-accounts registered and using up usernames that real customers might want. For what? I get... access to those people? Them added to a mailing list? No thanks.
Each user can change the reservation into a registration if they are interested in the service by logging in / using the service within X days from launch (up to the startup), else the reservation gets canceled from the startup and the (initially reserved) username is freed.
I think it's because it's an inherently selfish model - "Stop other people having the things I want"