The Role of the Pianist in a Jazz Group
The piano is classified as both a percussion instrument and a string instrument. In a jazz group, duplication of roles should be avoided.
As soon as two instruments take the same role (duplication) the music suffers.
Because the jazz group has a percussion instrument (the drums), a harmonic instrument (the bass), and a melodic instrument (the soloist), the tendency for the piano to duplicate the roles is always present.
I have always considered the piano as being a superfluous instrument in a jazz group. It is just not needed.
The other instruments are already fulfilling the melodic, harmonic and rhythmic roles. Due to its ability to be so interruptive, the piano's role in a group is then very sensitive and must be used with restraint.
What then is the pianist's role in a jazz group?
One must assume that all the members of the group know the chord changes. Hence, the piano is not needed to plunk down the chords for them. The bassist is already doing that.
The group already has a drummer so the piano is not needed to keep time and the soloist is already playing melody as well.
The pianist's role is that of a rhythmic, melodic and harmonic colorist.
As a matter of fact, all instruments are colorists. The pianist must strive to suppress the percussive aspects of the piano and make it, in a sense, "liquid."
Chords that are attacked in a manner that is too percussive, too loud, or too active, will interrupt the rhythmic flow of the music and distract the soloist as well as the listener.
Voicing too many notes will confine the soloist's choice of melody.