I am also a back-end developer and have found design knowledge to be very useful. Very often I'll be given a layout which breaks once I started adding code to

I am also a back-end developer and have found design knowledge to be very useful. Very often I'll be given a layout which breaks once I started adding code to it, a good html/css understanding will let you fix issues like this without having to pass it back to a designer. I can think of many times where design skill has paid off, namely a time when our sole designer quit leaving me to fill in for him, but I've found the knowledge to be extremely useful even if I only need it occasionally.