Should I replace the roof or just lay a new roof on top?

[Jacob Beg]

Hi All-

I am rehabbing my first multifamily 6 unit building with a flat roof. This has a torch roof, and I don't have much experience with these sort of roofs. 

The roofer has given me a price for removing the current roof and redo all over, or just lay a new roof over the older one. He says that in our jurisdiction it is permitted because the current roof is original material. The current roof is not in good condition, in fact there appear to be small holes, etc. The roofer seem very confident that there is no real need to tear up everything. Replacing everything is twice the cost. I am just not sure whether there there is any particular draw back in just putting it on top of the older one. 

Any advice?