Technically I would agree with you.
If you were there less than half of the month, but miraculously your bills were way higher, I don't think it's fair to you to have to pay that much more.
It's just like the analogy that someone else used: if someone washed/dried 100 loads of laundry, that's not fair to the person who didn't do any of that. (of course I would hope that they are getting their laundry done somewhere else or just buying new clothes b/c then they would be stinky
)
With my last roomie, her boyfriend was over ALL THE TIME. Even when neither of us were home. I didn't have a problem with it at all b/c he was a cool cat, but he ran up the bills like no one's business. I had a talk w/ my roomie about it and of course she got upset with me, but if he's there all the time (wasn't going to college and didn't have anywhere else to be) it is kind of like having a third roomie that's just always there.