32MB is more than enough for the 1024x768 res of the iBook
64MB PowerBooks never went past 1440x900 (ancient 5.3 17") or 1280x854 (the much better 5.6 15")
15" with 1440x900 (5.8) and 17" with 1680x1050 (5.9) allways had a sufficient 128MB

ADB wasn't really that bad at its time, clearly predating USB when everybody else was useing all sort of weird connectors for KBD or mouse.
Later it was more of a case of "if ain't broken don't fix it" as the keyboard make no difference in OSX-use. Under MorphOS one will notice that both "CMD"-keys send the same code resulting in the loss of "L-Amiga" (USB sends 2 different codes).
With trackpad it was probraly just easier to implement those 2 finger gestures over USB then ADB-protocol.

@namelegion
Unless that 1GHz is (near) freebie:

- Consider wether you want 15 or 17"
- look for PowerBook 5.6 and 5.8 (15") or 5.7 and 5.9 (17")
- 5.6 and 5.7 have a lower resolution than 5.8 and 5.9
- 5.6 has 64MB VRAM as standard, 128MB was available with the "Dual-DVI" builtoorder
- 5.7,5.8 and 5.9 allways come with 128MB VRAM
- all models can be upped to 2GB RAM, but the 5.6 (and older 15") are notorious for 1 RAM socket failing limiting you to 1GB
- 5.8 and 5.9 can take 2GB as single RAM-module

More info:
Everymac