Even if the singer wants the accompaniment, it may not always be easy. If the song is something a bit unfamiliar, an accompanist will have to edge their way into it slowly until they get the hang of it and have an accompaniment idea that will work. Easy to do on a guitar, not so easy if your instrument can only take a dominant role (e.g. tin whistle, which will be out of tune if played quietly).

Staying in the background to add subtle touches of sound here and there is possible on an accordion, but the only players I've known who could do it well were *very* experienced.