I have a five gallon hex, with two Otos, and two platies... I was thinking about adding a small air stone in the tank, is there any benefit to doing this, and is there any reason I should not do it?
the only question is "Do I really want the bubbles?" Based on what has been said, the airstone would be purely for your enjoyment....not for any real benefit for the tank.
Airstones bubbles can be for entertainment for the fish who like to swim in them often. It will also help agitate the surface water. It is also a nice decoration to add to a tank.
I'd tell you that if YOU want the bubbles for your own visual enjoyment, then by all means AIR STONE away,...but beyond THAT for the tank that you have described,...there would be a very minor practical use for it.