I'm not sure the concept of reason vs emotions is really up to date with neurology anymore. They seem to be more connected and equally necessary than previously thought in brain function, at least from what I've read.
Two examples that come to mind that illustrate this: I read an article about a male with a brain injury that resulted in a lack any emotional feeling or expression. His memory, perception, comprehension, language ability, etc appeared to be intact. Doctors studying his case wondered if he would be "super logical" in his thinking without the interference of emotions, but to their surprise they found that he had an incredibly difficult time making decisions, especially when one choice did not have a great advantage over another. He couldnt decide what color pen to sign a document with at work, and would stand for an hour trying to decide which breakfast cereal to buy in the grocery store, weighing all the options. We rely on reason to analyze choices, but emotion may be the spark that actually causes us to finally make a decision and act on it.
Emotion seems to be motivating in other ways. In an anatomy lecture about the thalamus, a professor said that one of its jobs in the brain was to scan outside stimuli and if it detected something unexpected or unusual, signal other parts of the brain to pay attention to it. The thalamus attaches an emotional sensation to incoming information, an expectation that something might be pleasant or harmful. If the emotional tag says "this is really bad, " the fight or flight response is activated via the hypothalamus even before the information is completely analyzed by higher levels of the brain. In other words, our emotional responses have a survival advantage in causing us to respond very quickly to threats, where as reason often takes too much time.