An additional perspective...
I fly out of the Washington DC area, and the wait at security lines can run anywhere from 5 minutes (Saturday am at DCA) to 1.5 hours (Friday 5pm at IAD). Having missed flights in the past because of the wait at security, I tend to overcompensate by arriving around two hours early, even on domestic flights.
By carrying on a small roll-aboard, I can get through security relatively stress-free, then have the option to standby (free on United) on an earlier flight if one is available. I couldn't do this if my luggage was checked.