Yeah, a good audio editor is extremely difficult to program. Just ask Adobe who recently spent a couple of years re-programming Audition for both Windows and OSX, and the program is still missing a bunch of features that were in the previously Windows-only version.
As to why we need an audio editor, there are lots of things that are simpler in a destructive editor instead of Sonar's non-destructive paradigm. I find that dialog editing is lightning quick in Audition, and only kinda fast in Sonar :)
And Audition's spectral editing (which other editors have nowadays) makes click and pop removal very simple and fast. Sonar is still a lot more cumbersome than a dedicated editor. It's just a matter of different tools for different jobs.
So while it would be nice if Sonar had various destructive audio editing features, I think it works really well now as it is: You can simply select a clip and fly it over to your editor, make your edits, and Sonar will pick up the changes. Given the complexity of doing an audio editor, I'd rather Cakewalk focus on core DAW functionality that can't be better accomplished with plugins or other programs. But I sure wouldn't complain if they did put in more destructive audio editing.