> why are they bad? <

Not bad! Not! Nooo!

I have a large collection of billiard, snooker, and 2-cushion balls and used to use them all, though for different purposes at different working venues.

I like snooker balls (2.125") because you can get sets in solid colors, thereby using an identical pattern of 3 or 4 in each hand, and rotate them in parallel or in opposition patterns. The colors make the patterns easier for viewers to follow when you're doing close-up or walk-around entertaining.

I also love joker balls. These are antique billiard balls with opaque swirled-colored surfaces. The game that was played with them is no longer popular, so you can only find them in the dead stock of billiard supply stores and only if you ask.

Sometimes you can find executive trac-balls in billiard-ball sizes. I have a swirly blue one that makes children exclaim: "it's the earth!" (I don't think these are made any more, but comparable translucent acrylics are sometimes available.)

A great trick is to palm and French-drop a swirly ball (track or joker) onto a flat set of 3 rotating balls. The gear-effect rotation of the top swirly ball is much more apparent than with a crystal, and is very impressive to viewers.

To experiment with and learn new multi-ball hand-rolling patterns I've used 1" or 1.5" acrylics (available from TAP plastics). When I found something interesting, I worked up in size with progressively larger balls until I max out.

Regular billiard balls have numbers on them, and most cue-balls have a spot, so I only use those when I'm in a bar and the conversation goes flat.

Billiard balls were my first hard balls. Before that I used 2.5" superballs, dipped in olive oil so they'd slide over each other. (That didn't last long.) Then I discovered magic shops, and then juggle stores began to carry acrylics.

Of course in the early early days we didn't use our fingers to rotate the balls, but used telekinesis instead. Before that we used rocks.