I've never ridden a bike with chainstays longer than 45 cm. I would love to take a Surly LHT for a spin fully loaded, everyone seems to love it's handling (46cm stays). Or better yet, I'd really like to try out the Sakkit Expedition with it's mega long 47.5 stays. But are those too long? If not, I would think they would be more common.
I've heard that the rear wheel starts to slip when climbing on dirt with the longer stays, and also be a slower climber. I really like stable handling bikes, so I'd be interested to find out at what point the negatives start to outweigh the positives.
Let's say you were ordering a custom off-road, 26" expedition touring frame. What chainstay length would you specify?