I've not seen any websites that require a Facebook account to log in, but my immediate reaction on seeing one would be to assume the person who implemented it doesn't know what they're doing and can't be trusted with my data.
Which has nothing to do w/ lock-in on 3rd party websites. It has to do w/ the relative security competence of those who choose to use FB Connect vs. those who choose email.
When I see a dodgy username and password form, what I parse is "I can't wait for them to email me back my password in plaintext, and then store it without a one-way hash."
That's not to say that they are the strongest professionals in the world, of course.