Plastic covers are used to protect the coil from damage and to keep the string from snagging the bobbins. They are used cosmetically to enhance the appearance of an instrument and normally do not alter the tone of the instrument. Metal covers used on Humbuckers and Telecaster rhythm pickups are used to reduce unwanted outside electrical interference commonly produced by transformers, light dimmers and bad wiring. Seth Lover used covers on his humbucker to reduce electrical interference and to cosmetically enhance the appearance of the pickup. The plating and cover material can alter the tone of a pickup. Nickel Silver is most commonly used on Gibson style humbuckers because it has less effect on the high end. Some companies use Aluminum and Brass which cut too much of the high end out of a pickup. Many types of plastics are used for injection or thermal formed covers. Some plastics used are High Pressure Styrene’s, ABS and Nylon’s. Several companies used thermal formed covers with the components epoxied inside. Older epoxies can eventually shrink and break the coil in side the cover. Covers are used to help reduce wear and tear to the coils and bobbins and especially keep the picks and fingernails from damaging the coils. Fender Stratocaster pickup covers have 6 holes to allow the magnet rod pole pieces to protrude through. This allows the staggered pole pieces to better balance the output of the various gauge strings. Through the years of playing in hot clubs, sweating and guitar cases with lots of humidity, the bobbin and pole pieces can absorb much moisture and eventually cause the pole piece to rust. Covers with holes in them don’t really help to much. Basically that’s why pickups are dipped in wax to help unwanted vibrations and feedback from loose parts but also to keep the moisture and humidity from being absorbed in the bobbin.