said it elsewhere, but: having the removable carseat is good for one thing: you're popping into the supermarket, DD has fallen asleep from being in the car, you pop the carrier into the cradle on the supermarket trolley and she stays asleep
Also good in restaurants if, like my DD, baby doesn't like being in the sling when I'm sitting down (it's my arms or nothing).
Still can't imagine wanting to wear the car seat, though. Travel systems weren't enough convenience for some people? (I don't do that very often, but eh, it's a nice feature.)