At my current airline sometimes deadheaders will take the JS to help out and give agents a hand on oversold flights. I think certain airlines such as 9E wont give you full pay credit if you take the jumpseat. At least thats what I was told by some 9e pilots, not sure if it applies to 3M as well.

I guess it would depend on the length of the trip, definitely a long trip would make you second guess riding in the soup bowl when you have a seat in the back. Not to mention if your ride on RJ's (crj200 mostly) bad weather/density altitude can dictate weight restrictions and the JS might be unusable due to it.