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Been reading all the comments regarding the V12 dizzy.
My theory, is related to the aviation industry and the use of magnetos. I don't have a V12 jag but the science behind the use of the venting dizzy is this. This dizzy is parked between two hot cyl heads. It is subject to extreme hot and cold cycling. A breeding ground for condensation. That's the main issue.
The effort to keep such condensation out has to just be enough so no moisture rests on any internal surface. Carbon tracking can form as well as flashover from dirt particles. So I predict that the designers needed to be able to continually vent the dizzy cavity, to both rid any moisture that may have accumulated from the previous run and maintain a moisture free cavity during the next engine run. The use of the gasket just helps keep the flow coming through the filter rather than sneak up through the cap seal surface. I would imagine a fair amount of air would be needed to keep fresh air moving through the dizzy and since there is a slight built in restriction through the filter anyway, any slight air drawn in would create a slight vacuum effect within the dizzy such that you don't have a straight through effect from tube to tube but more tube, cavity, tube. If it drew too fast, say, with no filter or a broken hose, it would take the path of least resistance and have no benefit to rid the cavity of moisture.
I dunno about all that. Sounds plausible to me.
But this is what I did 5.3L with GoreTex gasket
The GoreTex is very soft, so will most likely wind up thinner than the paper gasket.
I'm keeping my distributor cap system with a gasket and vent, but for a different reason that I don't see mentioned here. In my past drag racing days of old I remember that auto manufacturers like Ford and Mopar (early 80's) dealt with ozone generation (Ford called it Ionized Air) inside a closed distributor cap from all that arcing going on with the now higher (than before) voltages of solid state ignitions. Any condensed moisture (temperature change) in the cap mixed with ozone caused corrosion and thus poor performance. The answer at the time was to vent the distributor. I am only guessing that Jaguar engineers of that era realized that if other respected manufacturers had the newer hi-tension voltage problem in the V-8 engines that it could pose a problem even more so with 4 more arcs going on in the V12. Then the only question was to decide how to vent the distributor cap and Jag chose what I have on my '88 XJS, which is fine with me. If She's happy, so am I.
