Its just a standard non elemental tower that deals damage at a different rate than the others but is currently broken. Poison is not delivering damage like it should and not to mention dragon is supposed to take increased dmg from weakening + lower dragons attack from enervating. With research you will have both of those as well as around a 15 second duration for the poison to last. People think it sucks because of its low stats and slow attack rate but x15 stacking on a dragon can be a problem.
Just too bad they broke poison even worse than what it already was when they released the spell update.