If you're flying enough to earn high-level elite status (generally 50K or better) on a legacy carrier, or if you're flying internationally a lot, you're better off with a legacy.
However, if you're buying coach fares and don't have a good change of scoring an upgrade to F (silver or no status), I don't see how the legacies are better than WN. If you're not reserving far in advance, WN's model works to your favor -- reserved seats don't help if all you have left to choose from is middle seats.