There are advantages to a 69er off-road, mostly that it allows you to spin up the lighter rear wheel and still have rock clearence with the front wheel. And that is for bikes and forks designed to use such a setup.

There are no advantages to this setup on the road, especially when the bike is not designed around the smaller rear wheel.

The only way it would be even remotely sensible is if you were using a very fat tire (50mm+) on a bike that could not otherwise use fat tires, and you didn't need a rear brake.