maybe they'll break 200 average and not be stuck in the 190's....

I know several bowlers over years who just don't believe in using a spare ball. Many of them don't realize that the spare game is one of the most important aspects of the game.

Some bowlers don't want to carry that extra ball in their bag. Some bowlers believe in their ability to adjust hand position to flatten out the shot. Or, have another reason altogether. Though, the margin of error is lessen when the ball doesn't hook and/or the line to the single pin is a straight line.

I admire those bowlers with a sound spare game, plastic ball or not, it's an art to me. Seeing someone struggle to convert, or stuck just below 200 for many years when using a strike ball make me  ....

I think many bowlers would benefit with a spare ball, or at least give it a try for several months and see if this helps.