But, if the application become more complex using of a RTOS helps you a lot (keep simplicity, maintenance …).
My experience is that if a project becomes complex enough to justify a RTOS the '51 is not the right choice for that complex a project. I am not "against" the use of a RTOS, I merely do not believe that you need a RTOS to blink a few LEDs. Had this been an ARM forum, you would have seen me in the front of the RTOS advocates.

Erik