While mergers can be difficult for airlines, they are also a quick way for an airline to pick up limited resources that they could not otherwise obtain -- like gates at airports where all the gates are already leased out, landing slots at slot-limited airports, and the like.

My recollection of the WN comment on picking up the US or DL shuttle was that they wanted the gates and slots at airports like LGA, not necessarily the actual aircraft. WN would presumably use their own aircraft to operate their own Northeast shuttle. I agree that adding a new aircraft type to their fleet would be a huge undertaking and would probably cause them much more trouble than it would be worth. If they add a new aircraft type at some point, it will probably be a regional jet type of aircraft to allow them to enter markets where a 737 is not cost effective. I can't see them picking up MD-80s or 717s.