to prevent "corrupted" project files, such as http://forum.cakewalk.com/Sonar-X3-Trying-to-open-a-recent-project-that-is-failing-to-open-even-in-recovery-mode-m3111280.aspx sonar should perform some integrity check or "validation" before or after saving

Yes I agree. If possible.
 
But what would it validate? That it could open?
 
I mean the only way "we" know it's a corrupted file is it won't open. I don't think Sonar knows it! But we sure do!

well, it's difficult to say without insight into how the save actually works, but for example (a super simple example!) let's say there's a parameter that has to have an integer value between 0 and 127, but a typo in the code (sonar or 3rd party plugin) results in a value between 0 and 1270 (or say 0 and 12.7) being saved, on validation sonar would check that parameter value must fall between 0 and 127 and is an integer, detect that it doesn't/isn't, and raise some kind of error flag or possibly use some default that does fall within the range... so at least it could possibly prevent it becoming "unopenable"...  i'm sure there's lots of this kind of stuff going on anyway, as noel has  indicated, but yeah always room for improvement :-)

Sounds like an "expensive" operation, time and resource-wise.   However, as option and perhaps a separate process from simply saving and opening, and maybe resulting in a output log, sure, I can see its use!  
 

Each service pack or Sonar version will probably parse through each file in a different manner. In some cases it may try to fix issues discovered with old Sonar files. It gets a lot more complex though with third party plugins, Sonar cannot really predict their behavioir esp when plugins get upgraded over time. All Cakewalk can do it write defensive code. It's a miracle it works so well IMHO.

this sounds superfluous to me. i really don't want to sit through an integrity check ever time i press ctr+s. that would waste hours cumulatively over something I have never had a problem with.  User backup and safe mode are all you need.