After watching this video tutorial: https://www.youtube.com/watch?v=AyJsOoQKRjQ
I'm wondering what the whole deal was with the batteries. Apparently, you can resolder the circuit board inside the Rumble Pak slightly to make it batteryless. And there is no mention of the rumble effect being "lesser" when not using batteries, or anything like that; it seems to work just a swell with the power it gets through the cable from the console! This is surprising to say the least.
So why did it officially require batteries? I also remember seeing ads in magazines for third-party Rumble Paks which didn't require batteries, and I always wondered back then how that was possible. But apparently, Nintendo could've easily sold their own official ones without any battery compartment. But they didn't.
How can this be explained? Most likely, there is a logical explanation.