The idea behind the covers is to keep grit from getting into the coils and causing damage. The old PAF Gibson pickups, (and a lot of new ones), used beeswax to fill any gaps in the windings and keep the wires from vibrating.
Plastic covers would serve this purpose, but while you're at it, why not use metal to improve the shielding? This is why the chrome and 'gold' covers are made of brass. You get good attenuation of EMI from outside sources, but since brass is non-magnetic, there is NO reduction of the magnetic coupling between the strings and the pickup.
If you prefer the look of the uncovered pickups, go with 'em, but don't let some 'guru' try to feed you any crap about 'edge'. The type of magnet material, the size and shape of the magnet(s), the size of the wire in the windings, the number of turns in the coil(s), and the way the coils are positioned relative to each other.... there's a whole LOT of factors that are important to the way a pickup works. Don't get sidetracked by cosmetics.
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