You want to have Pro Tools running if you work with clients who regularly use it. That's not to say it needs to be your main program, but you want compatibility to make it comfortable for people spending money with you.
It's easier to say "Yes, I use Pro Tools" than to say "No, but if you do blah blah before sending the file, I can mix it in Pro Tools and do blah blah before sending it back for you to reopen by doing blah blah.
This is one of the things Reason got right in their new upgrade. I can now send a file with all the sounds used in it attached, so if you got Reason 5, but don't have sounds I do, you can still open my file.
FL got it with .rar files a while ago, but you still need the same plugs the user sending the file had.
Compatibility is the game. Once you're in a position where people will compromise their workflow to work with you, things change. Until then, you wanna have what your clients ask for rather than trying to tell them they're wrong and you're right...If those clients pay. I could buy Pro Tools 80 times over with the money i've made using it. I coulda made the same money without, but it would be made by inconveniencing my clients who expect me to use it.
A gym doesn't have to have treadmills, but how would you feel if you went to one and they told you to just run in place or get on an exercise bike?