Other people could use that time for e.g. watching the TV, but is that really any more a valuable way of spending one's time?
Also just because I make "skilled craftsman" type hourly rates (about as much per hour as a plumber) unlike a plumber I can only realistically get precisely 40 hrs per week. Not 39, not 41, but exactly 40 hrs at that rate.
Yes hrs 1 thru 40 I get about plumber income per hour, but as soon as I hit that 41st hour at home, I would have to hunt for a job and in this economy blah blah and with the flexibility required for a second job, and only wanting to work precisely one hour not 20 every week etc, I think I'd be VERY lucky to cashier at quickie mart for $7.25/hr, if that is even possible.
So unless you can actually do it, and you want to, don't assume the cost of a marginal extra hours labor is your regular pay rate. In other words the cost of an hours labor at $job during regular business hours is plumber-ish hourly rate, but at home after hours I cannot realistically earn more than a couple bucks per hour.