What is the downside to me purchasing a road bike instead of a hybrid?
The real answer is that for a 9km ride, virtually any bike will do the trick. It's hard to be uncomfortable on a bike (that fits you) for that short of a ride.

However, I'm fully in the road bike camp on this question. Hybrids are, in my opinion, a compromise with no real benefits. Most people aren't going to ride them off-road (and most road bikes can deal with short periods of offroad travel without problems). Cushioned saddles look great to novices, but divert weight away from your sit bones and are all but guaranteed to be uncomfortable should you decide to ride longer distances (for this reason, I can't recommend leather saddles highly enough, particularly Brooks'). The knobby tires that come standard on most hybrids have poor road  handling characteristics, and actually result in less traction on the road in wet conditions (bicycles cannot hydroplane).

On the contrary, a properly setup road commuter will offer you more hand positions via the drop handlebars (on the flats, the hoods, and drops) compared to just one. This will be invaluable for longer rides where your hands fatigue from maintaining one position. They can come with wider non-racing tires (I commute on 28C Panaracer Ribmo tires, and haven't had a flat in over 10,000km). And appropriate commuting frames come with rack and pannier bosses, and many will have racks preinstalled.

So while a hybrid bike will work, an appropriately-equipped road bike would be better in that it will be able to adapt to your needs should find yourself wanting to ride longer distances or join faster group rides.