Technically you don't! Which is why it gets pushed so far back even in my build. The thing people don't usually understand though is that the stats mythics give is no joke. Magic Penetration on rocketbelt and luden's is absolutely essential to killing squishies efficiently. On top of that both have either an active or a passive that adds to your burst damage. And in the case of rocketbelt, if you're having trouble getting into range or chasing its not only a dash, it grants movespeed. If you think other items are too important to miss out on situationally then I can almost see it being justified to skip the mythics but they are just too strong to ignore on most games