There are now 177 Hybrid buses on the road. Even accounting for dead buses (probably around 20) and with 35-40 assigned to Merivale (which is closed on the weekend), that leaves roughly 115 Hybrids available for service on the weekend. That's more than ample to cover all the runs on the #'s 1, 2, 3, 4, 5, 7, 9, 14, 16, 18.
I've also routinely seen Hybrids assigned to routes like the #96, 101 ( I know that interlining comes into play occasionally). Not only do they not save much or any fuel on those runs, but the electric motors are subjected to the wear and tear of Transitway/highway operation, for which they are not designed.
Pretty hard to justify the extra 250000$/per bus premium when the buses aren't being used on the runs where they can save the most fuel and in areas where less pollution is beneficial.
Operators and passengers prefer the Hybrids too, so it would be nice to see their usage maximized.
The Inveros are also the most costly buses in the fleet to run. They should be parking as many of them as possible when they have the opportunity in order to save money.