When you come down to it, Led Zeppelin was a trio too...instrumentation-wise.

For it to sound good, the bass has to have a good fat sound to fill in the rhythm when the guitar takes a solo. You'll also find that power trio guitar solos incorporate a lot of chordal/rhythmic stuff in it...again, to not leave a gaping whole where the rhythm guitar would be. I've read that Alex Lifeson (Rush's guitarist) will use a lot of double stops and stuff in his solos to fatten his guitar sound...again, so that the sound doesn't go flat when he solos.

Also, when you have Geddy Lee, John Entwhistle, or John Paul Jones as your bass player, there are no holes in the sound :-). For a power trio to work, you need a strong bass player