I don't really think it was a waste. She wasn't meant to be anything more than a bump in Calzona's relationship. She was used to break them up, she did. The end.
After Arizona is done with Leah, I hope that Leah and Arizona will be friends, or at least work friendship like Alex/Arizona are. I really liked seeing Arizona tell Leah something to calm her down. But I didn't see any "sparks" there, just a friendship vibe.
Unless...of course Leah ends up being Mark's daughter. In that case, ICK.
