Some hate it, but it does have its uses.

I like to use thin (2 mil) mylar on areas of the playfield that get more than average wear.  This is the ball shooter lane, and the ball is forced to change direction while at high speed because of the metal guide on the right.  This was causing a stripe to appear in the clear coat on the playfield, hence the mylar strip you see here.

I also put mylar in the orbits, and extra thicknesses wherever the ball drops onto the playfield.  Also, places where the ball cannot be seen are prime candidates for mylar, since these areas don't get cleaned as often as easily-accessible areas.