What's your commute distance? 15 pounds doesn't sound like much, but if you have an hour each direction you probably don't want that on your back.
Also, I wouldn't worry much about the laptop when crashing. How often are you just riding along and wipe out? Doesn't happen very often. But if the laptop has a standard hard drive and you have an incredibly rough road to commute on that might be an argument against using the cargo racks.