When you put a plug or switch into an electrical circuit, you always get more resistance. If the voltage involved was higher, this wouldn't be a big deal, but in most of these cars you're trying to draw as many amps as you'd be getting from household current (~3A) from a pack with a voltage less than a tenth of household current. I still wouldn't worry about it much. The added convenience of being able to swap out motors with just a pull of a pair of connectors, to me, makes up for the loss in current.