if you're going to try to do any design work that any sort of artistic element to it, the ability to create vector graphics (read: use the pen tool) is a must. no question.

i had no idea how important my illustrating skills were until i started working at my new job. we don't do full vector drawings, they come out looking realistic with textures and so forth, but i've got the pen tool in use at least 60% of the time. second to that would be text for inputting content.

edit: just because your final output is a raster/bitmap image doesn't mean the ability to work with vector paths isn't needed.