I play both bass and saxophone and sing backup harmonies in a 50'/60's/70's music (all genre- rock and roll, some country, lighter jazz standards, even some polkas) for mostly older people at RV resorts. These folks know what the songs should sound like, so we attempt to capture as much of the originals as possible so we can pass as a cover type band. Our lead singer is also a Johnny Mathis impersonator. With our limited personel- bass, keyboard (who covers the bass when I play sax), guitar and drums, we can pretty much capture the essence of the songs without having to be a perfect copy. We keep getting hired back and play about two gigs a week( we could accept more, but most of the guys don't want to work mid-week due to their day jobs). We are considered the best band of this type with these older retirees and are very popular- we are already booked for New Years 2012, 2013 and 2014 . These folks hire people like Wayne Newton and The Statler Brothers so they are somewhat picky on how their money is spent and the money is pretty good compared to alot of places I've played over the years. My point being, the songs don't have to (and can't) be exact copies of the originals to make people happy- but they just have to be close enough to sound like the originals . As long as I play the bass line that starts out "Stand by Me" or "Brown Eyed Girl" through most of the song, and the guitar or sax solos are recognizable, the drummer is coping the basic original style we are successful.
I imagine there are some bands that would require more perfect renditions of the originals, but we don't seem to have to be one of those bands to get work. I think an example of that kind of arranginging would be the bands that back up the original Doo-wop groups on the PBS specials. Those guys are all studuio musicians reading charts made from the original 45's.