By protecting our product the way we do, the amount of time it would take to crack is similar to the amount of time it would take to develop the new products the hard way.. the way we currently do.
If we released our application in unprotected .net form, it would be only a matter of time before our market share was decreased substantially due to the fact that we'd lose our edge of being first to market. We might still be first, but the second to market companies would follow very shortly after us, unlike now where we have a year or so or more...
There seems to be this arrogance that code written in an application is the only way the job could have been accomplished. So if a competitor wanted to duplicate your application, he would need to write it exactly like you did.
A couple years ago a huge portion of the Windows 2000 source was released. Did that instantly spawn new OSes and allow the competition to take over Windows? It's easier in most all cases to duplicate the functionality of an application by writing it yourself rather than trying to decompile and understand someone else's source code.
So the questions asked earlier of who and why people would want your source code is very relavent.
And yes, there are many many major applications written in .NET.