Well, a 2600 Y-cable wouldn't have any uses, and I assume you mean that the end that plugs into the system doesn't accept the additional CV pinouts, only left, right, up, down, and fire . . .
Were there C64 or Atari 8-bit games that allowed for such a device? That'd be my guess as to what a "2600-only" Y-cable would be used for. . .
No experience with them, though, I actually like the doorknob controller . . .