The starting point is NEC article 240.4. It requires all conductors to be protected against overcurrent.
The breaker in the panel upstream of the disconnect is intended to protect the conductors that run from the breaker to the disconnect. If the disconnect is fused, then those fuses are intended to protect the conductors that run from the disconnect to the Rooftop Unit (RTU). If the disconnect is not fused (and it is not required to be), then the upstream breaker is doing the job of protecting the conductors that run all the way from the breaker to the RTU.
For the sake of discussion, let's say the disconnect has no fuses. You are suggesting connecting new conductors to two of the terminals inside the disconnect, and running them to a new piece of equipment (the PE). What is going to protect those conductors? The only thing that can is the upstream breaker. Therefore, your new conductors have to be the same size as the conductors from the breaker to the RTU. That is a bit of overkill, since they won't be seeing as much current as the RTU will see. You might also have to upsize the breaker and all the conductors, so that they can handle the combined current of the RTU and the PE.
Is it legal? I believe so.
Is it a good idea? I think not.
Is Tom's idea better? Definitely.
