Same here, main reason is simply that I don't want the lights to go on when I'm not there, which would mean I'm in jail, dead, or otherwise not able to care for my plants, or worse, put out a fire. I live in an appartement building so have to consider the risk to neighbors.
I have an alarm set and usually respond within a couple minutes, but 20-30 minutes doesn't seem to affect my plants at all. I wouldnt do that during the 2nd and 3rd week of flowering though, but once they are female, I nor they care about "minutes".
Anyway, get a timer, they are only $5