So, you wait until you sit down and then look it up. How often are you faced with a "I must know what that word means NOW" situation?
.
Don't get me wrong, I'd love to have a dictionary on my Opus. However, having now used it for a while I've come to the conclusion that I'd far prefer ePub support and a separate dictionary, to Mobi with a dictionary. The benefits of ePub are compelling.