Not really, it just makes sure you are less likely to flatten the doner battery. It's quite possible to jump start a vehicle using a spare battery, I do it all the time, I keep a large tractor battery on trickle charge for just that purpose. It does tractors mostly and my digger cos I'm too mean to buy a new battery for it.
Haha nice, I jumped my boat engine with a spare live from the starter motor once when the remote ignition system packed up on the Thames! Closest I've come to hotwiring