i guess the primary reason why bands play covers is so they could get the crowds attention. And besides, the people would already know the song so it'd be easier to react to it either by headbanging or moshing which in turn motivates and inspires the band. Sad to say not everyone out there would know a band's original song so they'd just stand there watching the band scrutinizing their every move as they play originals which can be a bit scary for the bands up there because they dont know whether ur actually enjoying the song or the song sucks. I feel it takes a lot of balls to play originals on stage and salutes to all the bands that do so.