Plus the plying makes the yarn softer, more lofty, and even though the spin is in the other direction, because the two (or three or four...) plies are actually being twisted more because of being together. So that makes it even stronger.
So I haven't really been brave enough to do singles. But I saw a really beautiful example of Dudley Spinner's single ply socks while I was at the Weaver's Guild show, and I'm really, really tempted, now, to just do something like that. I'm using black faced Leicester, as it's strong enough on its own. Plus I'd dyed some really nice fall colors, and then over-dyed it again as the original was too faded for me. So it's really deep reds, greens, and warm browns now. Perfect for socks for me.
So I'm spinning the wool on my spindle and I'm very happy with how it's turning out. I'm also really ruminating over how to build the sock, as I'm not going to have a great idea of the gauge until I try it. Probably use #1 or #0 needles, and I'll likely be putting bits of roving into the parts of the sock that'll be under the heel and ball of the foot as that's where I wear them out first. So few of my every day shoes have heels that the back of my socks almost never wear out.
This should be fun. It's cool to *think* about a sock again.