I spend a lot of time on ledges in the Trapps and I see lots of rappel anchors. Most guides I know spend time reconfiguring junk anchors left by unsuspecting parties. Most of the very popular trade routes now have bolted belay/rappel anchors which I think is the way to go. However, there are still a lot of tree anchors of varying quality. Twice this season I've found tree rappel anchors with 6 slings on them. I've only been out guiding in the Trapps twice so I'm batting 100% right now.
When you find an anchor with 6 slings on it what do you think? My guess is that most people think "Oh, there are a lot of slings there so one of them must be good", or "all 6 combined are strong enough". The truth is that with 6 slings on a single tree it's hard to evaluate the integrity of any of the slings. That means 6 knots and 6 pieces of cord/webbing to inspect.
As a guide I'm always teaching people to ask the question "why" before committing to an action. If you don't have a good reason why you're doing something as a climber, well you probably shouldn't be doing it. So, assuming the tree/gear anchor is good, let's break down the "why" for 6 slings.
Question: How much load does that rappel anchor bear at any given moment?
Answer: Even if you hang 3 people off of that anchor at once the maximum load that anchor will bear is probably 750-1000lbs. Remember, this isn't an anchor that will experience big spikes in load, all you do is lean back and slide down the rope. One single piece of webbing or cord will easily bear that load.
Should we only leave one piece of cord at that anchor then?
We could add a second piece of cord to share the load and provide some peace of mind. This principle of redundancy is frequently employed in climbing and ensures that should one anchor fail we have a back-up in place.