I'll trade processor time for usability any time. As long as animations don't get choppy or the app starts to feel sluggish or battery drain skyrockets, I couldn't care less if an app is using a lot of processor time.

Actually, Skype is a good example of how to not do it. Having Skype run in the background decreases my battery life by a good quarter. Actually talking to someone over Skype can easily get the laptop fans going. Skype even freezes for a few moments every now and then. And Skype is still a tremendously useful app, so I will even take this abuse. (The new design of the Skype beta for Mac is a different matter, though)

Sparrow on the other hand is smooth sailing.