Advantages of mounting bottle under frame 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?
Other?  I would not count protection as the first.  The cage is a lever and some edges.  A water bottle is not exactly an impact attenuator.  And you have the chain ring to get past.

You pretty much only see a third on touring bikes. It is the least convenient location but it is a third location.  Three bottles or a frame bag and one bottle.  Pretty much impossible to access while riding.  Typically don't have the clearance in the  bottom position for a long bottle.

Someone commented on pull downwards.  Notice I used solid loop cage below.

I give center of gravity zero points.  

On a mountain due to a small triangle it is typically the second cage so naturally it is going to be more common on mountain.  Still the least functional position. On a mountain bike you can shake out a water bottle. I tried it on my mountain and gave up. 

My best use of the last position is margaritas on Critical Mass. You give up volume to insulated containers. That is a large frame and that is 70lb dog.