I see at least three potential problems here.
The first is that Delta will now be offering a more desirable Shuttle product than (and possess a potential marketing advantage over) US Airways for the vast majority of its Shuttle passengers. While a dozen passengers will enjoy First Class seating on the US Shuttle (and on many flights, there will likely be plenty of elites to fill those seats but that's Problem #3), the rest will sit in standard coach seating -- and Delta will continue to offer additional legroom in its Shuttle fleet. Given a choice for the same price, I'd take the extra legroom in a heartbeat.
The second is that US will lose capacity (4 seats per flight) with little potential for extra revenue. It is possible that by offering a guaranteed F seat for its Preferred passengers paying full fare that they might gain some incremental sales of full-fare tickets.
The third is that US likely has enough Chairman's Preferred members using the shuttles that contention among the CP
's for those F seats might lead to some dissatisfaction among those customers.
While I recognize the operational advantages of no longer having to maintain a separate Shuttle fleet, it must also be acknowledged that there is a cost to eliminating the higher quality of Shuttle service which many frequent travelers on the route have come to expect.