I like Chris Kinman's advice on many different topics, but sometimes I have to question him on others. The whole purpose of thoroughly shielding a pickup cavity is to create a "Faraday cage" around the area you're trying to protect from outside electrical interference. And with single coil pickups, it's the external 60 cycle interference that you're trying to keep away from your pickups.
What a Faraday cage effectively does, is to basically conduct the 60 cycle fields and send them to ground. In order for the "cage" to be effective, and in fact work at all, is for the entire surround of the shielding to be connected and grounded. Now, it's next to impossible to completely cover every opening to the control/pickup cavity on a guitar. The pickup poles obviously must protrude from the cavity so that they can conduct the magnetic signal from the strings. But, the surface that the pickups protrude from (the pickguard in the case of most Fenders), is connected to the other sides of the cage.
As to the shielding negatively affecting the tone of single coil pickups, I'll reference the work of Pete Biltoft at Vintage Vibe Guitars. Pete surrounds all of his single coil pickups with copper for the expressed purpose of shielding them against outside electrical interference. Anyone who has ever used, owned, or tried Pete's pickups (I have three) can attest to their superb tone - without any loss of frequency response.
Finally, it still comes down to each individuals ear. How does the shielding affect what you hear? Do you hear a loss of high frequencies as is claimed to occur by the detractors of shielding? I've got a pretty sensitive ear, but I'll be darned if I hear any. Perhaps if I had the hearing of a dog I might, but that's certainly not relevent. I'll bet that what most people will "hear" is actually what they "don't hear", and that's the annoying 60 cycle hum.