fd sounds much worse as now you can't jump to d characters.

False. The fd mapping would only apply in insert mode. Jumping to d only occurs in normal mode.

Oops - you're correct, of course.

I still don't understand how these mappings are good.

What if you need to type "fd" in insert mode? Like fd0, fd1, etc. Isn't that going to be a pain? Do you never have to type "fd"?

There is a timeout on multi-key mappings, so if you need to type fd0 you type f, wait 'timeoutlen' (1 second by default apparently), then type d0.

That's a pain, but it doesn't come up often. In return you get quite a lot of functionality (due to vim being modal) without having to move your fingers beyond the letter keys, which is quite relaxing.

Having fiddled with file descriptors, I could never live with such a remap. To each their own :)