Return movement types - 651 and 657

I am trying to understand a difference between 651 and 657. I have seen lot of post in scn and an interesting snippet caught my eye,

Movements types

"You could change the movement type to 657 (Returns

to Blocked Stock) which would be valuated. However, this would need to be

thought throuh in regard to - returns stock identification, warehouse

management, etc."

I understand 657 movement would result in valuated stock.. but what would be the disadvantage of 657 movement type when compared to 651. Could anyone please elaborate why one should not choose 657 (when we want return to be valuated)?