Have you ever sat down for Thanksgiving dinner, assembled your plate, taken a bite, and thought, This turkey is okay, but it's just too moist and evenly cooked? Me neither. Let me make a prediction: You will never have that reaction to a traditional roast turkey.
Here's the problem with turkey: above 145°F or so, white meat begins to dry out. Dark meat, with its connective tissue, on the other hand, has to be cooked to at least 165°F. How do you cook a single bird to two different temperatures? It's difficult at best, and downright impossible at worst, even more so when you consider the variation in shape and thickness of turkey meat, especially on the breast of a large bird.
Separating the dark meat from the white is the only way to nail the 20-degree temperature differential between properly cooked thighs and breasts. As a delicious added plus, separated legs can be slow-cooked to break down their connective tissue and provide a wonderfully silky mouthfeel.
As for the reasons to tie the breast into a cylindrical roast, look no further than Kenji's post on turkey-stuffed turkey from last year:
Even cooking. Because of its symmetrical shape, the turkey heats through along its entire length at the same rate. Nobody gets stuck with a dry piece.
Better seasoning. By removing the breasts from the carcass, you expose more surface area, allowing the seasonings to reach the space between the breasts, hence reaching the center of the turkey roll. Similarly, brining is more effective (though with low temperature cooking and an even shape, brining is wholly unnecessary).
Crisper skin. While it's possible to get crisp skin on this beast by popping it back into a 500°F oven for a few minutes just before serving, an even better way to do it is to sear it in butter in a big skillet on the stovetop--an endeavor that's reasonably simple with the breast's reduced size and more convenient shape.
Easier carving. With no bones and an even shape, carving this turkey is as simple as slicing a tenderloin.
Better gravy. With the entire carcass of the bird at your disposal, it's easy to make a delicious, very turk-ey gravy. I make mine by chopping up the bones, browning them, making a stock with aromatics, enhancing with some marmite and soy sauce, then thickening. Delicious!
Your family will like you more. Unless you're a control-freak kitchen nazi (I am).
The butchery itself is actually pretty simple, as the video above will demonstrate. Just remember to use a very sharp knife and to use your hands as much as possible.
After separating the appendages and deboning the breasts, I assemble the roast by stacking the breast halves on top of each other smooth-side-out, making sure the thin end of each half is aligned with the thick end of the other half. This guarantees relatively even thickness throughout its length. I wrap the cylinder back up in the skin and truss it with a series of half-hitch knots or. If that's not your bag, you can use several simple granny knots all along the length of the roast.
The easiest way to cook the bird is to roast all of the pieces in a 300°F oven on a couple of rimmed baking sheets fitted with a rack. Pull out the breast when it reaches 145° (tent it with foil to keep it warm) and the legs/wings when they hit 165°F. After that, crank the oven back up to 500°F, and about 15 minutes before you're ready to serve, bang everything back inside to crisp up the exterior skin (or you can sear the breast piece in hot butter in a skillet). All told, roasting should take less than 2 hours for a 12 to 15 pound bird, which is significant savings over a traditional roast turkey. Carve the bird, and serve.
When you take your first bite of juicy, evenly cooked meat, I think you'll agree it's well worth the extra effort of butchery. Well, unless the Swedish Chef is on. Priorities, people.