The value add for using FB connect is that if you do it right you can grow an app from nothing to millions of users a day, and have a top 10 (in their category) iOS & Android app without spending a dime on paid user acquisition.

The caveat is that the whole thing is a bit of a black art to get right. You can't just bolt it on and expect good things to happen, instead you have to build the whole thing inside-out. What I mean is that you start with FB Connect & Wall posts as the core of the app, and then build something that will get people to agree to the posts and have posts that have good viral properties.

(I used to work on viral FB apps up until a couple months ago, and did the above for two apps)