I would consider it worthwhile for any race as I don't want to waste time warming up in the first mile or 2 of a race. Typically I will warm up for at least 15 mins before a race. Even a 5k whereby my warmup is normally longer than my race!
Each to their own to do what works for them, the idea of wasting energy is laughable though.
Edited: 28/01/2013 at 14:17