One thing a piano (or harpsichord, and some electronic keyboards) has got going for it over the guitar: it can be tuned in non-equal temperament. I think this is sometimes done in Cape Breton piano accompaniment; I've heard it done with 18th century Scottish tunes, and it makes the accompaniment relate much more musically to the fiddle.

With a guitar you're stuck with what the frets give you, barring an occasional blues bend (impossible to use for voicing chords to pure intervals).