Theres another thing about slots that SLF and people who want to late board dont realise. Once issued they can improve to an on time departure. You may well get on the plane and then be told sorry 40 min delay due to slot, however, in the background ops ATC people are looking at re-routes and potentially filing a different route to the same destination or at a lower/higher flight level. This then clears the slot and bingo away on time, immediate start and push. Now if we've let Mr Cackwatch start running to the gate, we can't go, we might then get another slot on the new route whilst waiting, and the whole procedure starts again.
A slot is a capacity constraint on a block of airspace, if you miss it someone else gets it, (don't even bother to ask for it back), and you go to the back of the queue for the slot, regardless of the importance of your flight or destination, or length of the queue. Say you're evening flight is running late to Zurich (which closes at night, due to an absolute night curfew). If you wait for Mr Cackwatch your flight might not even depart because your destination isn't even open when you get there! Still want to wait for him? We are a service industry, but do we provide the service for 179 on time pax, or 1 late one?