I think its just for fun. cant really see anyone seriously using a 2 liter bottle for a bubbler when you could just as easily use a 5 gallon bucket and have much better success, you could put a clone in a 2 liter bottle if you really stayed up on nutrient changes for a little while anyway, but such a small resoiver would be alot of work and wouldnt even hold the root system of a very old plant you might get more interest if you only require a 2 liter bottle with no other restrictions im sure several probably have an extra clone that they are not using.