Why would you keep the wireless-N adapter knowing that the Slim has it built in? Wouldn't you be better served using that adapter for some extra TIV to defray the final cost?
I can't speak for the other poster but I happen to have two Xbox 360s- I bought a "spare" Arcade while waiting for an RRoD repair. I travel a lot so currently I keep one system plugged in at home and I keep the other system ready to go in a travel case. After spending $199 for a spare system I didn't feel like spending another $80 for a second adapter, so I have still have to muck with the adapter every time I travel. Once I upgrade to the new 360 I can keep the existing adapter in the bag with the travel system, so it's worth more to me to keep the adapter.
Sure, there aren't many people like me, but you asked for a reason why someone might keep the wireless adapter and I gave you one