Is there a HP advantage or for looks?
tommyo,
The chrome backplate is MUCH better looking than the regular plate, and will not develop the white aluminum corrosion that you will eventually get with the regular plate. Of course, the chrome version also has a smoother finish, so anyone wishing to justify it as a performance mod could conceivably make a case for slightly higher flow (how's that, 'd00d?).
Seriously, if you have the silver engine, the standard SE backplate is pretty close in appearance to the heads and therefore doesn't look bad. However, if you have the black engine, the chrome piece makes a big difference in appearance. Look at a '07 SERK, which has the chrome plate, and you'll see what I mean. If you're starting from scratch and buy the parts individually, you can actually buy an air cleaner that is better than the SE kit, with the chrome backplate and genuine K&N filter, for less than what H-D charges for the kit with the "el crappo" filter element they currently use. Check out the "Roll Your Own SE Air Cleaner" thread in the Twin Cam section.
Jerry