Why doesn't Apple give equivalences, like AMD does? They don't have to use Intel as the sole standard. They could build some sort of table, and give approximate conversions between PIV/Celeron/Athlon/G3/G4 for different tasks. For example, a 1.25 GHz G4 is equivalent to a 2.2 GHz Athlon or to a 2.5 GHz P4 for overall performance, or a 1.42 G4 is a bit faster than a 3 GHz P4 for graphics, etc. (I made those figures up).
It doesn't have to be accurate; just give the the average consumer an idea of the relative performance of the G4.