i dont see tool's bassist as a lead, and am very tired of listening to people say it. Tool's bass is nothing but roots and simple changes, it's just heavier pop really.
The only reason you think its lead is because its loud enough to hear, which shows where the instrument is socially. and the reason a bass solo will never peak a guitar solo is because if 50 differant songs had 50 differant bass solos, they would sound like 2. there is a slap solo and a finger work solo, and in the end they dont share the variety a guitar solo has with its effects, string work, chords, melodies, and general appeal.
its lack of actual workability that restricts the bass solo. if you think about it, bass trying to use effects to solo like a guitar does, just well, makes you sound like a guitar. and the band is not playing around you. it is fabulous if you can think that way but in actuality you are just playing silently to the listening ear to accentuate everyone else. so you are playing for them.
A guitar player can easily take rythm while a bass player solo's, its just todays players are not eager enough, good enough, nor does the audience want to hear it.