Downsides of Steam:
1. You have to be logged into Steam via their client all the time - as in on line.
2. Their server sometimes either refuses connections or goes off line. If that happens the software won't run. Not a situation I'd care to be in with a commercial client waiting to record something or working against a timetable. Not being able to run a game is irritating, not being able to run productivity software in an commercial environment could be a business killer.
3. I like being able to keep a copy of the installation files locally. That way, should Cakewalk cease trading tomorrow, I can still install Sonar and run it for the forseeable future. If I have to rely on software publishers making their software available for me to download every time I need to install, or for the publishers to have an account with an on-line service that verifies that software and gives me permission to run it, then I can not rely on being able to do that.
In short, the first version of Sonar that only becomes available via Steam or anything like it will be the first version I will not buy on principle and the version that loses Cakewalk my custom.