Because any less would be a giant ballache?
Why?
Well the seat on the Fiesta pops up and slides back on it's own accord. I don't have a kid, but given the choice between a 3 or 5 door car, I'd take the 5 door every time.
Much like I'm sure you'd get used to walking everywhere with no shoes on but given the choice, I'm sure you wouldn't do it.
I agree with Janesy 100%. Why bother making it more difficult for yourself for absolutely no gain? It's not like the OP's after an e46 M3 and we're saying no, go for a 330i because it has 4 doors.