If I can share what I do on other sites with my friends on Facebook (presumably people I know and/or care about), that's a value gain for both me and anyone adopting this, because I'm likely to use these other sites more often.
If, on the other hand, Facebook Connect isn't well adopted or it's a miserable experience, the Facebook really is just one locked in social network, and someone will come along and connect my profile across all websites. Someone other than OpenID which is the most annoying login system devised.
How is Facebook not already able to sell marketing information? Nobody wants to know which bands are suddenly becoming more popular (and where)? Or which movies are being looked forward to by which people?