Personally, I like it when they stack a card with fighters from one division, like the WW matchups in the GSP/Diaz fight.
If you have a title fight and then have four or six other contenders in the same division in two or three fights on the same card, that has a couple of benefits.
1) You get cover for main-event injuries - a top contender in the division was training to fight on that night anyway.
2) You get clearer options for the next title challenger - whoever impresses the most on the night can fight for the belt next time out.
3) Everyone is on the same schedule - you don't have to wait 3 months between the #1 contender winning his title eliminator, and the champion defending his belt against the last contender.