As you see, the buffer is kind of insurance policy

While that is true, I consider that reason totally insignificant.

In my opinion the reason for using a buffer is that, unless you use a derivative where the pins can be configured push-pull e.g. CY .. argh SILabs and Philips LPC, the '51 pins are a singularily bad choice for driving capacitive loads such as a cable.

Again, the pictures (yes, it has pictures) in "the bible" illustrate this very well.

In some designs one other reason for using (inverting) buffers comes up, which is that this makes the output low at reset.

Erik