I never count reps, and havent done so in years. I always go til failure. It just works best for me this way and always has. Plus I guess you could also look at it like this, if your having a bad day or off day and your goal is say 10 reps and you wind up not getting it, it kind of fucks with your head and the rest of your workout, and if your having a great day and say your goal is 10 reps then you stop however you could have repped out many more reps you kind of robbing yourself.
I dont know, i dont see anything wrong with it either way i guess, I just know I never count reps and always go til failure this way I know I have totaly exhausted my muscle no matter if I am having a good day or a bad day!