must always strive to make their work both efficient and effective.
I second that, however let me add: "not to the deteriment of the final products cost and functionality".
To those of us who are software inclined, an rtos that provides multithreading will surely simplify our job.
That defeats my add to the first statement.

Also, I have seen may projects where the use of a RTOS actually made the project more complex.

I am not against using a RTOS and have used them with other processors; however, I have yet to see anything where the '51 was the right choice of uC and a RTOS was justified. The '51 should be chosen for what it is good for, not as a multitasking, multithreading, multitom, multidick or multiharry processor, it was not designed for that.
I have done multitasking using the '51 using multiple interconnected (DPM or RS-485)processors, each running one task and that is the only way I would multitask/multithread the '51.

Erik