I see that some baitcast reels offer aluminum drive gears while still having brass pinion gears. Obviously this works or they couldn't do it with any real success, but I always thought that with dissimilar metals, the harder one would eventually wear against the softer one and it wouldn't last. Wouldn't it ideally be better if both the drive gear and pinion gear were brass, or aluminum??

I don't think there's that much stress and torque put on a reel for it to be a big issue heck some are brass on plastic my tennacy is brass on plastic iv used it hard for 3 years no issue. Now my rc trucks alum and plastic gears got chewed up first race lol.

Close tolerances help reduce wear as well while adding smoothness.

I'm not an expert, just a junk dealer that has bought a lot of this stuff. Alum bronze ( ampco) is used in all types of gearing. the alum gives hardness to the brass, brass is really a copper alloy which is very soft. Alum bronze is also not very expensive as compared to other brass or bronze alloys.