Well, those people wouldn't be affected. Even those people with a profile could still create throwaway accounts for those other subreddits.
There are a few sub reddits where people are sharing real life information, so maybe confirmed profiles would be beneficial for them. I don't know if that just adds a bunch of cost and not much revenue.
Part of what makes Reddit so good at what it does is that it doesn't try to be more social. By assigning very few pixels to users and making signup easy, the focus stays on the content of the discussion.
The idea of subreddits applies more generally. One piece of software, one account, many communities with different foci and norms. A more social forum could do the same thing.
In general, social networks tend to settle at establishing a norm of either pseudonymity or verinymity. The two don't mix very well; users either feel cloistered or overexposed based on the normative behavior they observe.
Sub reddits are just Usenet on the web but worse, rather than anything social.
Maybe if Reddit added a profile page with some social stuff (for paying users?) they could destroy Facebook.