alt Hacker NewsLenses have energy budgets, responsiveness concerns and actuator/sensor pairings through embedded processing. Therefore they need firmware. Firmware goes out of date and has bugs and improvements. Having the ability to configure a lens beyond simply selecting firmware is an obvious second use of the required interface.
Firmware doesn't inherently have more bugs than hardware, in general that's a consequence of the ability to update it more than the other way around. Here the need for an update-able firmware comes more from implementation a reverse engineered interface which might need fixes for compatibility with future cameras or updated camera firmware that only care about official vendor approved lenses (which consequently don't have a need for an update interface). However that still doesn't explain who would want things like auto-focus fine tuning or assignable buttons in the lens rather than the camera body where it already exists. Just because the USB interface is there doesn't mean that this use of it is useful.