I think that your chance of winning something should go up by 2 or 5% per wipe.
Why? Well, gold isn't really a reward. And when you are a healer or a tank (or even a dps, though generally it's less stressful for that role) and your team wipes 3 or 4 times and you get gold & gold, it's very frustrating. It's even more frustrating when it happens again...and again...and again...week after week. You stop feeling it's worth bothering with and thus you stop running it, increasing queue times and reducing Blizzard's interest in keeping the content alive.
This is mainly after frustration of 12 bosses in a row with bonus rolls and winning nothing. The group wiped many times on many of the bosses so I spent hours to walk away with...nothing of value. If I'm investing the time, I think there should be something worthwhile from the time.