Gertrude - You are right, but if that plane is half an hour late, and you come back half an hour late, someone will moan at you. Me, I used to book a plane for a 3 hour slot, buying me some time either end of what would normally be a 2 hour slot just in case.
I now have a share, and I wont deny it has cost me a lot more money (than I have really) than renting, but I'd have given up flying by now if I hadnt bought into a share. It has also opened up so many opportunities flying wise than you get with the constraints of renting and specific time slots.