There are still viable reasons to buy Pro Tools HD.
I think Pro Tools is popular because it is pretty much guaranteed to work (tried and true system). Not to say it does not have its share of bugs and crashes (Even adats would crash). But if I had the money, I probably would invest in a TDM system.
External controller support. This is a big one. Old school studio owners come from a world of knobs and faders. I think if they were told to replace their $200,000 analog studio with a computer and a mouse for all of the interaction. (Yeah right.). There are strides being made in this market Mackie, Tascam, and such but there is yet nothing out that competes with an Icon or Pro Control.
User Interface. While Pro Tools isn't known for its stellar midi editing, I found its audio editing interface great still above Sonar. What I liked:
Wide variety of editing tools (the region tool is one that comes to mind). Sonar default tool is for "moving" clips not "highlighting" them. This isn't normally a problem if your project is synched to tempo and aligns perfectly with the bar grid.
Sonar's method of highlighting clips works (the alt drag command), but thats the only good thing I can say about it. I'm not talking about highlighting clips with the snap set to musical time, Sonar does that well but if you are trying to determine tempo of a region, edit audio at a fine resolution, the Pro tools method (and Sound forge method) is a lot smoother. I find I can think a little better and audition a little better just having to use one hand to do something than both.
I am known to make a big deal over small things but IMO small things are the big things that really count.
Pro Tools also has very customizable gridlines. Sonar's grids just default to quarter notes. The Grid helps out also for editing audio, With it on you can easily see where something is getting off time. Fruityloops even go an extra step with gridlines and allow you to display swing. Sonar's grid method is a bit behind in times right now.
Another thing Pro tools allows you to display mutiple rulers in the arrange pane. Also handy.
Another thing is Pro tools method of auditioning regions for offline plug in processing is very logical. Sonar don't play what you highlight for offline processing. It instead just plays the beginning of the clip. This makes offline processing a bit useless.
A lot of things too aren't neccesarily things Sonar can't do and Pro Tools can but the problems is usually one program might have easier ways of doing something. Pro tools repeat command comes to mind. Just select a region click the repeat command and enter the number of repeats and voila. To do this in Sonar I'm am taken back to my days in Calc II (no it is not that bad). In sonar you first must copy the region to the clip board, choose paste, enter the time where you want to start, enter the number of repetitions, enter the track where you want to start, and choose the proper interval length. Pro tools way of doing this task is extremely easy, where Sonar's method is extremely easy to do something wrong. Things like this are things Sonar needs to seriously consider. Most programs have dedicated repeat, duplicate commands as well as copy and paste commands.
On a good note since I started using Sonar back on 2.2, Cakewalk has made significant progress. Back in the 2.2 days when Sonar was compared to Pro Tools as far as audio was concerned, I felt Pro Tools was definitely better. Since then Sonar has rewritten their audio engine (less gaps, better mixbus), added clip muting, clip based effects, a better mixer, nudge commands, and a lot of these features Sonar benefited a lot on implemented them. Clip muting comes to mind. I used to drudge having to silence clips to arrange parts but now clip muting makes this ridiculously easy.
I think Sonar will catch up with Pro Tools interface in due time. Currently Pro Tools is still a little better on the audio side (to me for editing), but I get by with Sonar now as most of my problems have definite workarounds. But too many workarounds usually mean there could be a more direct way to get something done.