Interesting in that two of my good friends (a couple) who regularly fly UA out of DEN (he's a 1K and so is she via the MM companion program) flew Spirit RT to LAX and said "never again". She said that by the time they paid all the ancillary fees the $200 RT was more expensive than it would have been on UA, and they didn't enjoy the experience at all.
He had learned not to fly F9 (Frontier) last year when he saved a few bucks on a leg from SFO-DEN and had to pay for his carryon.
Business travelers who are mandated to buy the lowest fare could be hurt by this.