A valid viewpoint to be sure.
However the step back should provide the necessary protective distant, and if does not, then, by all means, "block that sucker".
When the party to do the block/conditioning strike steps back he should block the incoming strike if the latter extends into "his zone" too far.
Part of my point also is that in arm pounding you are 'watching that strike" it is your only concern. In T'ai chi they say 'let your eyes grow soft" . this is seemingly quite a jump from 'glare in the eyes with fast hands".
What is happening is allowing your eye to 'disuse' the focusing poing point of the eye ie: the fovea and allow full vision.
So I submit "block that sucka" is more appropriately and completely applied in prearranged Kumite.
Note that even when you make a mistake and forget the prescribed block, something occurs to the mind and some one of your quiver full of blocks jumps out and does the job.
If Arm pounding is to be completely practiced as a conditioning exercise, both parties had better be on the same page.
So the first lesson is properly establishing the correct distancing so that, in theory, the step is a block, and the conditioning strike is used for that purpose and the striker can pay attention to hitting different points on the arm being conditioned.
You ALL better know what page the other fella is on before you assume it is safe to just 'step back' and start trading punches.
Most of the time you can tell just by looking what is likely to happen: someone with an attitude, somene who is overaggressive, someone who wants to look especially good (he thinks) by showing his partner up, someone who is really quite anxious to hurt you, and he, my friends, will try to hurt you of bo the 'to' and the 'fro' on Kontikitai.
Of course, i suppose it is easier to forget all these complicated concepts and just block fast.
Or, you can watch your partner's arm extension and depth of step carefully, then lay the lumber on various point on the arm.
But, Sensei GEM is right, until all the "protocals" mentioned have been established----just protect yourself, of course.
"All Enlightenment Gratefully Accepted"