No hard statistics, but it's my perception WN does more plane swaps than most other airlines; with all the equipment being 737's, this may be easier for them to do than AA, DL, UA, etc.

If a plane is delayed by a mechanical, they often seem to pull another plane off of its scheduled route, then perhaps do the same thing a couple of more times (rolling swaps?) to give the mechanics time to fix the broken equipment and get it back in service for a later flight.

Someone with more hard knowledge than I can comment on my perceptions, which may be totally inaccurate.