This is a clear cut on hit build. The early magic damage is both unexpected and cheap. Surprise is always a tactic to keep in mind. The reasoning behind Malady is simple. Even though it uses ability power, which does add on to Ashe's ultimate by the way, it cheaply, and therefor quickly adds more magic on hit damage along with what we are really after, the drainage of magic resistance. After the build is complete in late game it doesn't matter if you switch some of these items out for other items.