Yep, you're right... I think the only Orang library used is pushbuttons.
I tried to change the three refences to the pushbutton library to direct hardware control. It doesn't work, and I don't know why.
DDRB &= 0b11101111;
PORTB |= 0b00010000;
// Always wait for the button to be released so that 3pi doesn't
// start moving until your hand is away from it.
I'm pretty sure that it almost works, I just don't know enough about AVR programming to make it work. But that change saves over 500byte of memory.
It seems to me that the pushbutton library could be changed to much simpler macros that would use less space.
A quick look at Pololu3pi.cpp show that it's basically just a wrapper for the Orang line sensor array. Unless the compiler optimizes it away, it mostly just takes up space. You guys don't market the 3pi to noobs, so directly using the QTR library would seem to make more sense to me.
It might not be worthwhile to make the changes now, after it being that way for so long. At the same time, there are a lot of people (myself included) who would still like Arduino to fix the odd pin spacing, even though it would break compatibility with current shields.