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.