Well my reasons are:
1. Off road would be nice, dirt bikes are a lot of fun.
2. Wheelies and other stunts at much slower speeds and reduced risk of death than on my Ninja, and also cheaper repairs.
3. Canada has a lot of crappy roads and you just can't get full use out of a Sportbike a lot of times.
4. I would ride a dirt bike in much worse weather, as long as their was bare pavement I would go for a rip, but I won't roll the ninja out with salt and sand all over the roads, and snow banks.
5. Just to be different. Having a normally dirt only bike plated and tearing up and down public streets sound like a lot of fun to me.
I want to keepp the Ninja for the longer hauls and the nice canyon roads, but a bike like yours would be a great daily commuter and night time play toy, and like you said if switching tires is all it takes a weekend in the dirt would be a blast too.
The Dual sports are all heavy and de tuned, I do want a real dirt bike.