There are a good many reasons that guitarists release solo albums while they’re in a band. The number one reason is that they are frustrated singers—their vocals aren’t good enough to be spotlighted in their bands’ LPs so they must find another outlet in which to express themselves. The number two reason is that the songs are self-indulgent, so the band has rejected them.
For the most part, John Frusciante is an exception. His 10th solo album, The Empyrean, will be officially released tomorrow, the same day that Obama gets coronated. I found one track (“Unreachable”) on YouTube that’s pretty listenable, I must day. The vocals certainly annoy me less than Anthony Kiedis’ do and the track has a Beatles White Album kitchen table feel to it:
Perfect example of this is Tom Morello as The Nightwatchman, one of the greatest guitarists of our time and yet can’t carry a tune in a bucket.