I followed up by viewing several other pieces by groups which share this instrumental combination. I included in this four-piece groups that include a lead singer who either doesn't play an instrument or does so only rarely.
The guitarists often play rhythm (i.e. chords with several notes) and leads (the melody) at the same time especially when there's no singing. Depending on the style, the effect can be pretty cluttered. It's often difficult to figure out exactly what notes the guitarist is playing, especially if he's employing distortion of one sort or another.
It there were another instrument playing notes in the range of the guitar, the result would be a mess. However, the four strings of the bass are pitched an octave lower than the four lowest strings of the guitar. So long as the bass player stays out of the guitarist's range, only the guitarist has to worry about "saturating" that register. Here is Jimmy Page playing leads and filling in the holes while bassist John Paul Jones stays out of the way. Too bad Robert Plant can't sing the melody as he did in the recording. He doesn't even attempt the end of the chorus, leaving it to the audience!
In these trios, drummers often play on every eighth and most sixteenth-notes, especially in fills at the end of the phrases when the guitarist is playing longer values. Some of them (like the drummer in the Hendrix excerpt) play so much they seem to be "filling" all the time. If there were more instruments this would be intrusive, but in this context it works.
Bela Fleck and the Flecktones apply the same principle. In this case the soloist, Fleck, (named by his parents for Bela Bartok) plays banjo. This band's arrangements are all worked out so the virtuoso bassist, Victor Wooten, plays higher notes only at times when he knows it won't muddy the texture. If you check out other of their pieces you'll see they really get around, stylistically. Fleck has been nominated for Grammys in more different categories than any other musician.