Cool - though as much as I love Logic, I can't really brag about Amp Designer. IMHO, it's like pretty much all the others - some patches/models are fun and inspiring, it can occasionally sound absolutely right for something, but most of the time it's average - and then really poor for other things. The high gain models in particular are the worst I've heard in recent years.
Personally, I'd say factor it in as a cool bonus and a little extra something that you can probably get a few cool sounds out of, but just don't buy Logic hoping that you'll be able to sell your Guitar Rig license or your amp...