I'd say that by doing this they will gain more users to G+ than they will not gain for Gmail or other services due to concerns about G+.

On the other hand, being able to sign up with an arbitrary email address has always lead to issues once/if you switch to Google Apps and intend to share local parts.

Google has no obligation to provide G+/GMail-less accounts and I'm sure there is a lot of technical and business reasons for coupling them. As it's their service, that's what we get.