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.
That's true, but I just feel that's not the Google's way of doing things. I mean, they never forced you to use their other products in the past, so this comes as a surprise to me.
Of course, it's their company they can do whatever they want. That doesn't mean we can't judge them.
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.