It because Mew is technically the "father" of all Pokemon and contains all of their DNA. He probably just needs to be able to see the Pokemon before he can attack with their moves.
I have one copy in my Darkrai/Tornadus deck, I mean c'mon guys! It is basically a double junk arm with almost no drawbacks, you just may want to run a super scoop up or two in your deck to bring it back up when unneeded.
My favorite Pokemon are Charmander and Umbreon. I like them because they are really awesome looking and dark and fire just happen to be my favorite types
I expect Groudon/ Terrakion to be at around a tier 1.5, because like Shino Bug Master said, it has such a huge advantage due to weakness in this format