One of the best sessions I ever went to, in Hughes' Bar in Dublin, just had all the musicians playing the tunes without harmony at all. Well, there might have been occasional drone notes from Gabriel McKeon's pipes, I can't quite remember. If tunes are well played in unison the harmony happens inside your head. I've never heard anyone strumming along to Bach's solo cello suites. ;-) Most of the time in those there's a single melody line only, with just a bit of double stopping here and there, but the harmony is all there in your imagination. Strummed or plucked instruments "doing the backing" were what finally stopped me from playing in public a few years ago as my hearing went down and I struggled to pick out and latch on to the lads playing the actual tunes, so I suppose I'm biased.