I hear ya; but this isn't a one-or-the-other. They could draft the next Roger Clemens this April, and he's not going to help the 2018 big league rotation.
If you can get a guy like Milone on a minor league deal, and you can get comfortable with the injury, What's the risk? Why not take a flyer on the chance he can bounce back and be a #5 with a 4.50 ERA? Heck, he'd be our #2-3 if he could do that! If he sucks, you release him.
From his standpoint, we would represent one of his better opportunities to get some major league starts...assuming no one wants to give him a major league deal this off-season; and pending any major rotation upgrades (holding my breath ).