And autoCAD, and MS Project. Visio. Exchange... and that proprietary application that's really popular with X.
Linux isn't really fighting against usability (yes, it does have some issues there), it's fighting against an entrenched user-base whose needs are tied to existing software. Porting Adobe might help for some, but there are still plenty of other things out there.
In the same vein, though, for a lot of users it's more than enough. It's got email, web browsing /w flash/java support (facebook and games), word-processor, spreadsheet, some basic games, and now even some stuff on Steam. Getting Adobe moved over would help add to that, but I'd imagine that if Steam ever gets their Steambox out there with a killer title (another Left 4 Dead, Half Life, etc variant would probably do it), then adoption will increase quite nicely as well.