I'm wondering if there are any advantages to mounting ones water-bottle on the bottom of the down tube, near the ground, versus on top of the tube.

I'm mainly interested in the advantages, or lack thereof, in the context of a road riding.

It seems like if you are riding a mountain bike or riding off-road, it could protect your frame from stumps, branches, or anything else that you could land on that is taller than the bottom of your down tube. Also, I could see it helping to keep the center of gravity of the bike as low as possible, for mountain and road.

Are there any other advantages to mounting a bottle on the bottom of the down tube on a road bike?