While he's been one of the go-to goblin typal deck commanders, he's also useful in any goblin typal deck, period
And while the discarding of energy may slow some players down, it will be harder for opponents to knock this legendary dragon out, especially because Rayquaza's special ability reduces any damage dealt to him by grass, fire, water, or electric by 20
These two work best with cards that let you dump all that extra mana that green cards make into draw spells like Prosperity and Blue Sun's Zenith
It is only the red Welder effects that want discard outlets outside of the black ones and so there is a premium on the black cards that do it
After much consideration, here is my list (which, unfortunately, I know I should have included Tainted Pact on, but I did not, sorry)