Thank you Mark for expressing the situation perfectly. Back the 20th
century Sydney had a big system with lots of hills in it and smaller trams
with only two bogies per tram, so having all wheels powered was necessary.
Today the tramcars have three or four bogies per car which makes them more
expensive to manufacture and more bogies powered makes it even more
expensive. So they try to get by with a minimum number of powered bogies,
unless a system has a lot of steep hills to deal with (e.g. Prague) when
they don't have a choice. If there is only one excessive grade in the whole
system then the design engineers will look at ways of reducing or bypassing
that grade in order not to have an entire fleet that is up-specfied just to
deal with that single grade.

Tony P