However, there's much more to Siege Dragon. If your opponent controls no Walls, whenever it attacks, it deals 5 damage to each creature without flying the defending player controls. That's a pretty sweet ability. This is no Balefire Dragon. But at rare, that's still pretty powerful. It was decent in Limited and especially strong in a Sealed pool. Of course, that hasn't mattered in a long time.
Siege Dragon may not have been Standard-playable, but there have been Dragon Tribal Commander decks that found a home for him. It may not be much above a bulk rare, especially when it was competing in Standard with a far superior card in Stormbreath Dragon. If Siege Dragon had haste, though, this would've been a Standard-playable Dragon. Alas, it did not. Still, Siege Dragon does have its uses in obliterating ground troops. It gets the job done.