Specifically I felt that going through the YC application forced me to think through questions that any other investor/incubator/accelerator (and even some customers) would also have.

Additionally, since the answers I had to those questions didn't pass muster with the partners I can either shrug it off (YC is extremely competitive and even they admit they make mistakes all the time) and take the rejection as additional motivation or I can take their decision as truth and take a closer look at my current business model to see what can/should be tweaked. Either way it can be helpful :)

Of course it would be nice if the rejection emails were more personalized, but I just don't think that's possible with the number of applications they receive.