1) Bowling balls do get worn out over time, you can only extract oil and resurface so much before there's just not much left.
2) Always remember, you don't need to purchase a ton of bowling balls to bowl pretty darn well on 90% of conditions.
3) I don't know about you, but I usually win a jackpot shortly after purchasing a new ball that pays it off, so I never even notice the money I've spent on it! Heck, buying used DVDs costs me more than buying bowling balls if you factor in the money I make back using those bowling balls.