I am of a different opinion. If a comma by itself is the same as ,0 then it is very consistent, at least with the examples we have mentioned so far and it matches up with what is written in the descriptions of some functions. In some functions, a parameter that is omitted (i.e., just a comma, no value) is the same as setting it to zero. Until today it never dawned on me to equate "omitted" = 0 but now it seems obvious. I also never realized it was used in other functions, like you found out with AVERAGE. Doesn't seem to be universal, though.
SUM(B2:B15,) is the same as SUM(B2:B15,0). Adding a zero does not affect a sum so both are the same as SUM(B2:B15).
AVERAGE(B2:B15,) is the same as AVERAGE(B2:B15,0) which is not the same as AVERAGE(B2:B15). Averaging in an extra zero affects an average.
The example of DURATION is 0 weeks, 0 days, 0 hrs, 1 minute, 0 seconds. That last comma in DURATION(0,0,0,1,) is for the seconds, which does not change the duration because it is 0 seconds.
If you feel it is a bug or a mistake or inconsistent, please use the menu item Numbers/Provide Numbers Feedback to inform the iWork team. That is the official channel for reporting problems, bugs, suggestions, etc.