Well, how many balls you need really depends on the level of bowling that you are doing. To me, if you only bowl recreational leagues (1 or 2 times per week), I would say you need no more than 3 balls, one for medium to heavy oil, one for light to medium oil, and a spare ball.
If you bowl in several different houses and travel to tournaments, however, I can see the justification in carrying a large arsenal. The basic premise is that you never want to walk into a center and meet a condition that you are unprepared to shoot at. Now, we all know that changes in surface of the balls you have, and changes in speed, hand position, etc, should allow you to find a line on any shot. Still, having more options in terms of what you're throwing may give you more room for error and/or maximize your scoring potential. That's what tournaments are usually all about.
Personally, I have an arsenal of eight balls (1 for heavy oil, 1 for medium heavy, 1 solid for medium, 1 pearl for medium, 1 skid-snap ball for medium, 1 for medium-light, 1 for light, and a spare ball) How many of those balls get used on a regular basis? As is often the case, I can use just about any of them to score on league nights. The others are specifically designed to give me more options at tournaments.
I guess I liken this to golfers. There are many good golfers that can use a 7-iron for every shot during a round and damn-near break par. So why do they carry the whole bag? Well, those other clubs match up just a bit better for certain shots, giving them the chance to be more precise and score even better. For a bowler, our 3 and 4 ball rolling bags are our clubs.
I don't find any harm in people carrying large arsenals. To each his own, as I say. The only time I laugh is when a see someone who has an 8-ball arsenal in which every ball is drilled the same, prepped the same and designed to work on the same condition. That just seems foolish, but oh well : )
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I can't bowl 300, but I can bench 345 : )

I provide the muscle for the Fellowship of the Saws
