The only downsides I can think of are that the warranty you get with a demo bike might be somewhat shorter (not the full 2-years but the remaining part of a 2 year and 3 months warranty), that there will be some wear on the tires/brakes, and that the belt tension might be too tight as I suspect most dealers don't bother to adjust that during the demo period and it is known that that belt tends to shrink and get *very* tight in the first few months of leaving the factory. Most here have mentioned they had to adjust the tension down; having too tight a belt is not good for the drivetrain (bearings, the belt itself).
Some think demo bikes are better due to potential early-life defects having manifested themselves and having been fixed for you already. I don't support this idea - I'd rather know what issues my bike has and how they are resolved, and in all honesty, if a bike is expected to have many issues to be resolved within such a short period I would probably not consider buying that brand at all. Most Zeros should not have issues, so why rely on a demo bike to have potential issues resolved... Chargers are one of these things, I've heard, but from what I see they tend to fail at several thousand miles, not several hundred...