One professor put it that there’s three basic types of design students: the engineers, the visualists, and the practicalists. Fortunately, you don’t usually fly solo, so there’s many kinds of people in the team.
Even though I tend to think mostly the practical and engineering sides, sketching and visuals are probably the most important skill for a designer IMO - I think when people meet a designer, they expect him to be good in visualizing things. That’s why I’m working hard on my sketching skills right now. You need to be able to give that ‘wow’ feeling to clients, and that’s hard to do without good concept images.