IMHO there's an obvious reason not mentioned: due to their position on the dial and convention IV and VI would typically be depicted upside down, increasing the likelihood of confusing them—particularly with a semi-literate populace.
"Somebody thought IV was not easily understandable because it resembled VI..." (in paragraph 2)
So why use a numeral at all? Well, there's always those people who will think that it is obviously wrong not to have numbers on a measuring instrument. Also, in early modern times, clocks were expensive items and expected to be ornate, especially as they were not all that good at keeping time.
if the device is upside down it is evident and I don’t even try to read it, I right the clock or take off and restrap the watch.