All I know is that it opens me up for good hard riding later on. If I warm up, later hard efforts seem easier. If I go 100% from the time I click in, I feel like crap for the rest of the ride.

When I'm strapped for time, I do something similar to what the various training DVDs like Spinervals or CTS do. Start off with 5-10 minutes of easy riding. Do 1x2 at a lactate threshold and high cadence with 2-3 minutes of rest in between. Recover for 3-5 minutes and go at it.