this seems to be a common fear of google +. Using google + or related services doesn't automatically/magically open up your entire life to the whole world.
The privacy controls are (IMO) much simpler than facebook, for example.
For myself here's the reason. An email address is an extremely important part of a person's identity for all kinds of online activities, banking, shopping, government services.
Facebook is a huge security and privacy risk. But it's possible to wall it off in its own cesspool and it won't affect every other aspect of life.
If someone is using a gmail address for important stuff, it's scary to think about linking it to something that's anything like Facebook.
That's the same reason I'm really aggravated about iGoogle going away. I don't want to have to use some 3rd party replacement and have to give them my Google password in order to have my gmail, google calendar, etc, all on one page.
With iGoogle, it's all in Google's hands, who I obviously have trusted enough to have an email address there.