I've played bass guitar is sessions, and that can get some funny looks, like when I used to turn up with a solid and battery amp! As long as you know what the other accompanying player(s) will be doing, it can work.

I do think that guitar (or in my case bouzouki) accompaniment works best if the player knows the tune well enough. Just doing the three chords (or even two in minor keys!) isn't all there is to do. Mind you, sometimes that works well. The right hand work can be equally important to add something to the overall performance.

The main thing to remember (as TtR says) is that the tune is in the hands of the melody players. If it's getting sloppy, then the accompanists could minimilise (a chord every two bars?) to help pull it together.