Proving one is human takes human effort. Given that, I'd rather do something marginally useful than something completely pointless.

It would be cool if they released the gained data as open source, but that might compromise the service, and I guess someone has to develop and host this thing, so keeping it to themselves is fair enough.

The only issue is the conflict of interest (they benefit from giving me more captchas), but they don't seem to be abusing it for that end. They sure abuse their position of power by hellbanning you when they suspect foul play, but I doubt that it's because they want you to do more work.