"If it isn't obvious why, you can think of an unmeylinated axon as a leaky pipe ..."

With super-stretchy rubber walls, so you have to move great barrels of water in or out to change the pressure.

Myelin's other big purpose is to make the pipe walls stiffer (lower electrical capacitance), so the axon's voltage can be changed by far fewer ions. That gives the axon fast propagation without needing more ion channels than it can afford.

(I suspect capacitance drives myelin's shape and size. I bet evolution could come up with a thinner low-leakage barrier, but physics dictates that the only way to reduce capacitance is a thick layer of big, greasy molecules. It's the same reason we electrical engineers wrap our data wires in a thick layer of hydrocarbon, when a thin layer would suffice to keep charge from leaking away.)

Thanks for that augmentation to the metaphor, Dan. I'll have to remember that one next time I try to explain this stuff.