Oh wait, i get it. every symbolized string is added to the global symbol table, so you can basically make rails devour incredible amounts of memory by sending it new values. neat.
I vaguely recall somewhere that Matz commented about how he felt that he was a good language designer, but not necessarily a good language implementor. I've always felt that MRI should be treated as a reference implementation, and the fact that there aren't more alternate implementations of Ruby is a (maybe sad?) reflection on the Ruby community.