Some good discussion on my last post - I've been enjoying the comments. Thanks to everyone.
One commenter, Laura, remarks that in general she agrees with me, but has been knitting Jared Flood's Girasole, and it calls for sk2po, and that it looks better in that context than the s2kpo she would usually use.
I've had a look at the pattern - I can't see the details in the pictures, but it definitely seems like a visible lean may well indeed work well in the diagonal lines. I will try to get a closer look.
Northmoon also suggests that in the context of the Swallowtail, the sk2po does provide a nice little 'cap' that closes up the leaf. Evelyn Clark, the designer of Swallowtail, is a smart woman, and I'm sure she chose it deliberately. I perhaps wasn't fair picking on her or her design specifically, but it happens to be a great illustration of the lean of the decrease.
I do know that I see the sk2po used significantly more often than the s2kpo. And most of the time, when I see the leaning one, I automatically replace it with a vertical decrease - and it looks the better for it. It makes me wonder if the s2kpo is, like the ssk, a reasonably new innovation/invention/discovery/unvention. Is it possible that there are patterns out there that predate it? Is it possible that if the designer had been familiar with it, they would have written the pattern differently?
All that having been said, I'm now revisiting some of my favourite lace books to with a new eye - to see which double decrease is used, and to consider in which situations the leaning version might actually work better.
This is one of the great things about the blog - the discussion. I'm loving having my world expanded by getting to know new people, hearing other opinions, and learning new things. Keep commenting, world! Thanks!