Dick, old chap, making music is about making music, a collaborative thing if more than one person is playing. If the role of an accompanist is to make the singer keep pitch or melody player to keep time, then good music is not being made. Well, it may be good music in certain contexts, a teacher teaching a kid grade 3 fiddle perhaps, or Gareth Malone succeeding in getting a big fat postman to hold more than a note or two. But, generally, if you are having to strive to keep someone else in time or pitch, it's crap innit. We've all been there. It is not your role. It's your role to listen and make hundreds of tiny decisions in your own playing every minute, derived from your listening, and that goes for singers/melody players/accompanists in equal measure. Add a task on top of that, like keeping someone else in pitch or time, and the whole thing stops being good music and becomes one big tribulation. And it shows to the listeners and it feels horrible for the players. Except for crap players, who probably think it's great.