ProTools has basically been the industry leader in DAW software since the induction of digital recording. Because of this, it has become somewhat of a namesake when it comes to recording. Additionally, most schools still teach using ProTools and most major studios use it to some extent still (not saying exclusively as some others have started to become popular like Logic, Cubase, etc.). ProTools does have GREAT support for pluggins though. I know I ran into some issues trying to get EZDrummer working with Audition and that works with PT.
ProTools a few years back started falling behind, especially since they required specific hardware for its use, however, since ProTools 9 release a little over a year ago, that has disappeared and is no longer an issue with ProTools.
I actually used to use Adobe Audition (and still do for some things) but switched to ProTools. I did the switch for a few reasons:
1. It is what we used in school and I became familiar with it and could use it for assignments
2. Industry standard (I wanted to know it to help better chances of doing audio for bigger companies (video game companies, etc..))
3. I ran into issues with Audition not supporting my plugs.
4. I became accustomed to PTs and its features and REALLY like them
It is the DAW for me but not for everyone. What kind of recording do you do? Just home things? If you're getting along just fine with Audition, and your just recording at home or in your bedroom, there really probably is no real reason to switch.
