The “swirl” is just a part of the pattern of the foil sheet cards get printed on, so in terms of rarity its only as rare as how often cards end up being printed with the swirl being exposed in the holo part of the picture. While aesthetically more pleasing, I don’t think most people would go out of their way to look for it or pay extra for one… although I know of at least one personal friend who cares enough to do so.
The Team Rocket set has some nice swirls (and base 2 as well) Always a nice touch. Dark Machamp or Dark Dragonite with a well placed swirl really adds to the appeal of the card.
I just acquired this Typhlosion 17/111, this is the only card I can remember ever seeing with two swirls right next to each other. Pretty cool in my opinion.
Were the holo foil sheets consistent across a print run? Would be cool to see a full sheet of it to determine how common it would have been to see the swirl.