Besides the break from the double tax, there's also the time issue. The ATF reviews each transfer separately and they must be done sequentially. So, in the case of a non-C&R transfer, the dealer applies for the transfer and waits for ATF approval. Once it's approved and the transfer to your dealer takes place, then you apply for a transfer and wait again.
Last time I checked (just for curiosity's sake - I don't collect NFA guns), the wait time was four or five months per transfer.
Well we don't rent pigs and I figure it's better to say it right out front because a man that does like to rent pigs is... he's hard to stop - Gus McCrae