minecraft-java-edition
I didn't found information on the wiki about the dragon head (waited some time now).
I own one of this heads and wear it, but it doesn't seem to serve any purpose as far as my experience went. Except for my friends being jealous that I own one (maybe that's the purpose :P).
Does it provide any advantage (like the pumpkin head which makes endermen totally neutral)?
No, you can't combine the poison with stuff at the moment, but it might be a planned feature.
But you can create a splash potion out of your poison potion. Because you can throw a splash potion it can be used against other entities. Just beware, skeletons and zombies don't take damage from them. To kill these you need a splash health potion. Source
While your command works in principle, as of 1.8.8 the game is unable to render many items in the head slot, including flower pots.
However, as part of the new Snapshots/Upcoming version 1.9, Mojang Dev Grum (Erik Broes) has worked on rendering all item on entities and published a world showing off zombies wearing items, including flower pots:
Sadly, this will still only display the item as if lying on the ground, rather than the rendered version of a flower pot (the tile entity), and it will be placed at the back of the head (like the flowers in the screenshot above), meaning it will float above the armor stand.
Also note that the Equipment tag was split into ArmorItems (Feet, Legs, Chest, Head) and HandItems (Mainhand, Offhand) to accomodate dual-wielding, so the command would be
/summon ArmorStand ~ ~ ~ {ArmorItems:[{},{},{},{id:"flower_pot",Count:1}]}
Best Answer
You were correct in saying that except for your friends being jealous there is no point. It's just an awesome trophy (or killed one of your friends who had a skull). Also, when powered with redstone it will open and close it's mouth repeatedly.