Refaced or not ? My views on refacing of wind instrument mouthpieces

The sound of a mouthpiece:

is mainly determined by the internal shape of the mouthpiece and to a small but important degree by the material of the mouthpiece.

The playability of a mouthpiece:

is mainly determined by the facing curve.

Refacing means:

changing the facing curve and adjusting the thickness of the side rails and the tip rail.
Often, the table needs to be flattened.

A good refacing:

will not change the way a mouthpiece sounds unless there is a large change in tip opening.

When is a refacing needed?

A mouthpiece that has been played much will be worn and need a refacing.
An old hard rubber (and plastic) mouthpiece can be warped and need a refacing.
(Metal mouthpieces are stable with time but will also get worn in use.)
Often, a mouthpiece of a respected brand is badly finished, and will benefit from a refacing.

(Many good refacers also offer
repair of bite plates and shank cracks as well as changes in the baffle and the chamber. The latter will change the sound of the mouthpiece

This falls outside my definiton of a refacing.)

The conclusion is:

Old and used mouthpieces will almost always play better after a good refacing.