This is rather topical with me at the moment; as I am immersed in arranging for small and large ensembles...and expect to be doing so indefinitely! As I have said many times before - because I believe it to be true - the guitarist's role always depends upon a given context - the approach that is taken - whether improvised or premeditated (ie arranged/orchestrated) depends on what role needs to be fulfilled. Needless to say, playing guitar - as with any instrument - is not done in a vacuum without consideration given to everything else.
Take a small Jazz ensemble with just three horns (and a four piece rhythm section); which represents a fairly typical line up. Jazz harmony - as you may know - really starts at the level of sevenths - which are four note chords. So, it can be a challenge to get just three horns to play a harmonised 'line' (the top voice) and sound like Jazz harmony. One solution is to introduce the guitar in order to provide a fourth voice. Now we're cooking! of course, in doing so, the guitarist is transitioning from their role as part of the rhythm section (comping - which is improvised around a chord chart) and being an integral part of the orchestration. Not only would this normally require the guitarist to 'read' the line that they must play, but also they have to accept the same demands as any sectional horn player in regard to articulation, accent, rhythmic presentation and so on. But still, this is a useful role for the guitarist and one that should be looked at. In fact, there is no reason why the guitarist - in this role - cannot add their own 'chord melody' to what the horns are playing. Now that would really fatten up the sound! But I don't feel that it can be just improvised because that is taking something of a chance with the overall sound - it must be thought through and played the same way each time as part of the arranger's conception.
So there you go - guitarists can have a role to play in the orchestration and thought of as an extra voice to exploit.
