The bikes in a Demo Day are not usually a dealership's bikes. If it were, it'd be used as it would also be a transfer from the manufacturer to the dealership.
So, the real question is would you buy a Dealer's demo unit and that is going to depend on the dealership and their internal practices. I wouldn't pay new money and I would expect a new bike warranty or better than. Otherwise, just buy new.
I wouldn't buy a demo day unit. It's not just one demo day with those bikes. It's all the demo days, across the country.