Erik, if things have names we can communicate better, can’t we?
I agree with you, if the application requires only message passing from a single interrupt you’ll not need a RTOS. It can be found a lot of examples even in olds BBS.

But, if the application become more complex using of a RTOS helps you a lot (keep simplicity, maintenance …).

On the other hand, I think it's not a good option to use a level interrupt for catching a 5 us event – what can be done in 5us with a 8051 at 12MHz (4us is required just to return from interrupt routine). We must also notify here that not only clearing/setting EA(ES…) are disabling the interrupts but other interrupts routine too. So, how to catch a 5us event if we don’t know when it arrives. (Pranav I do understand - that post was just an example – and it was good)