I do not say do not use a URTOS, what I am saying is : "If you need real time you can not use a real time operating system". Sounds illogical, doesn't it. The problem I have is that a real time killer is called "real time". If you have no time critical operations it may be an excellent idea to use a URTOS if you use a processor capable of handling it. There may even be cases where the expression "real enough" may cover your needs.
Especially the URTOSs with a full complement of "device drivers" can be a godsend since they, while not letting you run real time DO take care of the time critical stuff in actual real time whilke saving you the trouble of coding the hardware interfaces. Such URTOSs, however will overload a '51.

HOWEVER so may so called "embedded programmers" that think everything is a PC derivative believe the world comes to an end if even a simple application is programmed without a URTOS. That is where the real abuse comes in.

I have gained more in the '51 world NOT using a URTOS than by using one. I would, however, not dream of proposing a cellphone of todays complexity without an OS.

Erik