It's a fair point, but by the same token it's usually easier to teach a backend dev good UI design principles, than it is to teach a mediocre frontend dev good

It's a fair point, but by the same token it's usually easier to teach a backend dev good UI design principles, than it is to teach a mediocre frontend dev good programming principles. There's also the fact that for a lot of the kinds of work we're hiring for, there's a creative team producing design documents that the developers are working against, so they don't actually need good UI skills generally (although it certainly can help, particularly when the designers produce things that don't actually work in practice).