I can't for the life of me see where around 1000 passengers a day at
PML would go to or come from (that must work out at around 20 boarding
or alighting passengers per train in the peak hour), and would have
thought that Gallions Reach or Royal Albert would have less use than
Beckton Park but still more use than PML.

Actually that's 1000 "travels" a day - since most of the passengers
are using it twice a day. Actually I can easily see 500 people using
station daily - after all, there are no local shops in the area, so to
get food or booze people need to go to Stratford or Bow.
Also, when people cannot get DLR tickets at Stratford (ticket machine
is broke, or something) - DLR "agents" usually tell people to buy
tickets at PML.