The upside: you will develop the ability to create things that the communication barrier between excellent developers and excellent designers make very difficult.
The downside: You will have to be an expert in both to keep up with the expert designers and expert developers you will work with later in your career... and every company you work for will try to pigeon-hole you (as design and dev are generally different teams with different management).
Your standard added value to the company will be in your efficiency. You will work at 1.5x the rate of your coworkers (as you will design with development in mind, and develop with design in mind). You will be golden every once in a while when a project comes up that requires novel design AND novel development (you will save the company a ton of man-hours in meetings).
Having both will generally move you to the top of the resume pile when you go job searching, but they will hire you as one or the other (and you have to be sure they will let you do both).
If you don't want to devote your life to it, I suggest you specialize in one.