I wear a 661 core saver what amounts to probably 70% (mostly trails, some moto when I can) of the time I am riding. The back coverage is what I am mainly after, but the low profile fit and cooler feel is what I mainly got it for as I need all the help I can get cooling down in summer humidity!
The other 30% of the time when I ride, I use my Dianese safety jacket upper body armor that I originally bought for downhill mountain bike racing. It is, by far, leaps and bounds better back, chest, and shoulder protection than the core saver and it has some killer elbow/forearm protection....but it's bulky. I wear it when we're out riding some really technical trails or when I am riding something entirely new to me (trail wise).
All that said, about two months ago I was out riding with my 10 year old nephew and went down. It was a stupid crash, as I was more focused on my nephew who was a bit out of control and did a little whisky throttle action. I wasn't paying attention and low-sided slow speed to my right, landing squarely on my right elbow first. End result was a slight break/crack of some bones, Popeye elbow/forearm look from the swelling and about a full months worth of meds, icing, phyiscal therapy and being off the bike! Moral of the story, sometimes.....errr, maybe all the time.....elbow protection is a good thing in addition to what you normally wear!