I went to a wedding in February. They had a live 5 piece
string and piano setup, basically providing the 'soundtrack' to the event; background music, the walk up the isle, leading in and out of cake cutting, toasting and other stuff. Generally a classical feel but with a few reworked pop ballads that the bride had chosen. It was indoors in a wedding hall and they were using in-house sound system, but they weren't the house band.
I was impressed with their skill at playing and mostly (thinking back) how they helped the overall atmosphere without pushing the fact that they were there, you know? Being discrete? They weren't there to provide entertainment like, "Hey we're the band, lets dance!", it wasn't that kind of party. I suppose you'd need a different style of group for that, or one that can do both, ie. knows when to kick out the jams and when to lay back! A well choreographed event.