I rarely fly anymore, since it's gotten to be such a time consuming and hassle filled ordeal. When I do fly, I just don't carry much of anything on other than the stuff nobody is going to hassle you about (clothes, a paperback book, etc.) Then when I get to my destination, I buy the essentials that I didn't carry on (toothpaste, shampoo, a new swiss army knife, etc.) I tend to go to the same destinations over and over, so I leave behind what I've bought so it will be there next time (works when visiting family, which are most of my flights these days).

I really can't think of much a small pen knife could do for you in an airplane emergency situation. Maybe cut a jammed seatbelt - although I've never seen any seatbelt jam, ever. Anyway, you could wiggle out of a jammed seatbelt if you needed too. It's not like you're going to carve an escape hole through the fuselage with your Leatherman Squirt.