Anderton
LpMike75
Using both X3 and PT 11 HD, my opinion is, if you are writing or creating music, X3 wins. With all the included plug ins, effects and synths, I would rather mix in X3. However, if doing any kind of post production or audio editing, Pro Tools is on a different level than X3 and is almost a 'no brainer'. So it all depends on what you are doing.
I agree with your post for one big reason - format compatibility. But if I was spotting for TV and audio-for-video was my bread and butter, these days I'd choose DP over PT.
But you're right...I'd still create music in Sonar 
Craig -
Ignoring format compatibility for a moment, the big problem I have with Sonar is the lack of navigation, snapping, cutting keybinds. You just cant navigate and edit as fast in Sonar as you can in Pro Tools, due to lack options. There is the "snap to landmark" function in Sonar which is a little helpful and on the right track, but still not in the ballpark of where you need to be for fast post or editing work.
In addition, the file management in ProTools is superior. Pro Tools you have the option of pulling files and saving files in several locations in one project, which is really helpful when getting files from other people on a harddrive and only wanting to return the "new" files in your collaboration.
I have put in feature requests to the bakers for some of these options, which could make Sonar a viable option in Indie post productions. Which would be awesome, given that you don't need Sonar "HD" to get surround busses :)