On a practical note, if the cart is in the aisle behind the passenger, and nature calls, the FC lav is the only realistic option. My flights are typically 2-3 hours and the use of the lav is not necessary for me. But, apparently, others are not so fortunate. I certainly would not deny them the use of the empty lav simply because they have to walk by my hallowed seat. Besides, that way they can see that I am important and sitting up front. The FA's shouldn't allow a line to form though.