So you want to dispatch an aircraft and in the event of a problem that prevents them landing, they should use their reserve fuel and decided on their own alternate and work out a route to it, because the dispatcher didnt want to build it into the flight plan. If they dont need to go to an alternate the fuel is not used, so you are not going to make much of a saving by not giving it to them in the first place (apart from a slight cost of transporting extra fuel). Hey what do I know anyway. I have only been in this game for over 20 years.