I'm the biggest Leone fanboy on this board, but to say he was a very very steady reliever is extremely misleading. He had one excellent year this season, previously he had been below replacement level for two seasons. Sure, you could attribute some of those struggles to injury, but as a pitcher if you've struggled with injury before what's not to say you won't again? He could be just as bad as he was the previous two seasons or just as good as this year, such is the nature with non-elite relievers. We'll probably find another of his ilk this season.
When you can trade a non-established guy like him along with a lottery ticket soon-to-be-converted prospect for an average (or better) starting OF with CF experience with control, you don't think too much about it.