When it enters, it can cause your Birds to deal a ton of damage to a creature to get rid of any problem creature your opponent(s) may have
Sure, its a deflating game-ender, but the uses of it are narrow enough (only in five-color decks, for one) that it might be safe out in the wild
There are many more reasons beyond the mana difference why I would rather a Nihiri than a Pack Rat as a discard outlet in most situations
so, while you can get cheaper versions of these cards, the most desirable variants are the ones designated as Secret Rare
Another interesting tech Pokmon to play that might help a bit in draw is Noctowl from HeartGold - SoulSilver
It was positioned as a holiday expansion , outside the mainline series, making its arrival even more special