A lot of folks have been focusing on tone differences between fingers and pick, but the main reason I personally like playing with a pick isn't so much about tone as it is about the physical up-and-down motion. I generally try to play downstrokes on downbeats and upstrokes on upbeats, a la Carol Kaye, and sometimes this motion seems to help me feel the groove in a way that I don't get playing with fingers. There are some songs that I (almost) always play with a pick because they (almost) always feel better that way -- e.g., Grateful Dead and Beatles -- but mostly it's a matter of how I'm feeling it at that particular moment, and I choose to play with or without a pick on a whim.