This is it. In high school statistics, I tried to use my dad's HP-48g, which I dearly loved (in large part thanks to RPL---this was fairly soon after I discovered Scheme and dc). This didn't really work: the teacher would explain how to do something (automated statistical tests, for example) on a TI-89, and I'd have to spend a whole bunch of time trying to figure out the equivalent.
This was a pain, and it was compounded by the fact that the HP-48 didn't have the same features as the TI-89, so I was frequently out of luck. The teacher wrote his homeworks and tests with an eye towards what was doable without too much effort on a TI-89, and I imagine that the TI people chose the featureset for the 89 with an eye towards what people would want to do in high school classes, so using anything else was an uphill battle.
(We also had occasional homeworks in Excel, and I ran into the same trouble trying to do them in Perl. It didn't help that I was very far from an expert in either Perl or the HP-48g.)