It does let a little competition in for a big bit of expansion you can make, especially if you're stuck in a corner by no gates being available for rent.
it trumps (i hope) the AE system, where when you build a terminal, it must be as large as all rented gates AND all rented gates are immediately returned to the open market.
in the future (next age), there will be a cap on how many gates you can rent at an airport, so expansion beyond a certain point will require terminal construction.
edit: Fixed the edit route bug. sorry about that.