50 amp to 30 amp stepdown.

The adapter isn't a stepdown, it's only hooking up the 30 amp to one leg of the 50 amp.

A 30 amp connection has three wires - hot 110v, common return and ground.

A 50 amp connection has four wires - 2 hot 110v legs, common return and ground.

A 50 to 30 amp adapter connects one of the two 110v hot legs, the common and ground.

The 50 amp service is protected by a 50 amp breaker. So, you cord rated for 30 amps can pull 50 before the breaker pops.

On 30 amp service you are protected by a 30 amp breaker.

Other than those occasions when there's only a 50 amp outlet, I only use the adapter when the 30 amp outlet looks marginal or damaged also. There's no significant reasons not to use the adapter, but eliminating the extra connection does have some merit.

Personally, I would not use the adaptor unless I had to. If there was a issue with the 30amp connection (damaged/worn/loose) I would then use the 50amp with my adaptor.