My purpose for a filter is cheap insurance. With or without a filter Im always careful - but again,no matter how careful you may think you are,sooner or later there will be a mishap and THEN you will say "if I had only used a filter"... But dont focus on the filter issue - anything could happen. Anything from handing your camera to another person to fumbling with trying to remove a lens from your bag - skies the limit with what 'could' happen. Point is,sooner or later it WILL happen,and when it does you WILL be p*ssed and heart broken.
I use actual insurance on all my gear, costs me a whopping $50 a year for nearly $5000 worth of protection.
I think it's silly how much people fret over filters, if it makes you feel better, use them. There's no "right" answer here. I don't use them partly because I've seen them give me issues in the past (ghosting and flare) and because I find that the front element is usually easier to clean than filters are and I'm lazy
I've used lenses with horribly scratched up front elements and they produced great shots, it's seriously not as big a deal as people think it is.