Should passengers be boarded in cases such as these, so the aircraft can depart at short notice in case a revised slot is allocated? IMHO this is an airline policy issue - and the policy should be clearly stated to the handler. A major carrier I used to work for (and susequently handled as a customer) had a clear policy for this situation - pax to be boarded.
Sitting on board for an hour or two extra is not a pleasant experience for the passengers, but at least they can be kept up to date with the situation by the cockpit crew. And it is not such a frustrating experience as the missing if the improved slot outlined in the first post.