If authentication is completely optional, and provides next to no features, is it okay to dump it in a preferences tab?
I would suggest that if something is so inconsequential and apparently not useful that you can just "dump" it somewhere, it probably doesn't need to exist.
I think this quote by Antoine de Saint Exupéry applies (you may have heard it):
... perfection is finally attained not when there is no longer anything to add, but when there is no longer anything to take away ...
If there's no real features behind the registration wall, then why bother at all?
If it's to gather email addresses and some user information then theres nothing wrong with that.
If a user is entering their information, just to sniff around and see what's behind the curtain, then they'll use a inactive or inaccurately entered email anyway.
Once beyond the threshold though, it they're convinced of the service, product, or whatever, then they should have the option of confirming and verifying their contact info.
There should be a reminder or similar to the user about creating a profile or authenticating once they're in. This will essentially create to subsets of users, those who are registered, but not authenticated and those who are authenticated. You can then target communications to both groups, giving important info to the authenticated ones and selling points or reasons to authenticate to those who haven't.