In fairness the early Fender pickups were not potted at all ... but then they were never meant to deal with modern high gain amplifiers and high stage volumes. In fact, an un-potted pickup has a more open, detailed sound: due to some inherent microphonic action when unrestrained pickup elements move like a microphone diaphragm. But squeal is always a threat, and most people would prefer the peace of mind of wax potted coils.
An interesting fact is that the harder the potting material the more 'sterile' a pickup sounds ... thus epoxy potted units suffer no microphonic problems but sound cold. Wax is a good compromise in my opinion and allows pickups to be 'unwound' and rewound again more easily.