On a THS, I only take 1 ball and my spare ball. I'll alternate them every 2 or 3 weeks when I get bored or if I'm bowling league at a house that 1 ball matches up well for me (summer league) I'll just stick to that.
Don't get me wrong, having the wrong ball on the wrong day has cost me some pins, but having 5 balls to choose from all the time would cost me a lot more than a few.
I also bowl on a modified shot, and I tried at the beginning to take 1 and a spare, but they get squirrely quick. It's nice to have a smooth solid or a late breaking pearl to alter the reaction.
I, am however, a member of the ball wh*re club as well. I have a bunch sitting in many places of my home. It's really just a waste of money at this point, but it's my money to waste