I know exactly what bolt your talking about, Im surprised that you eclipse still has it, my car and many others usually lose it by now!
Anyways, The bolt is their for a purpose but is NOT needed THAT much! I have to periodically re-tighten my belt, but its no biggie.
When I have to retighten the belt, all I do is just loosen that first bolt, take a crowbar and lean it up against the pulley, therefore pushing it back against the belt and at the same time tighten the bolt real good!
PS: Another good thing to do is to put a piece of wood right where the other bolt used to be (this will prevent the belt from becoming loose again!
Hoped that help! Good luck!