The science officer is a key player in many missions. His in depth knowledge of physics, chemistry, biology and geology (not to mention history, archaeology, politics, phrenology, and about a dozen other 'ologies) makes him a valuable source of knowledge on a mission. Add him to a landing party and you'll find your options opening up. He's also very popular with the ladies. No, nobody knows why.
The science officer may be drawn from the operations pool of officers. A command officer may act as a science officer in a pinch, but it's really not the same.