Mike,
The removal was at my request. It added about $2000 to the cost but I had a ton of Eternabond tape up there plus at least three coats of a previous rubber roof coating. The roof looked pretty bad with large cracks in the coating and I was not sure if it was still watertight or not. Turns out it was and there was not one speck of water damage.
I'm glad I spent the extra money to have it redone right down to the wood covering. Now I know it is sound and will not have to worry about it anymore.:)
In your case, if your roof is not leaking and is in good shape, there is no need to spend the extra money on a tear off. I would just have them go over your existing roof. I believe that they guarantee it the same either way.
Barney