Herrera would count as a surprise though I agree you'd be giving up defense, even if they do have other OFs who can play the position, none of them are as good right now. It's possible Altherr could be though, if he was out there every day. I mean, Herrera had never even played it and look at him now.
The problem with trading Quinn and/or Cozens is they don't actually get you anything. They are at best a third piece behind a Kingery or Sanchez and another good prospect (Cozens has little value right now; Quinn's is questionable due to his durability). So long as we are talking about the attractive, controllable #2-3 pitchers who are all going to fetch a high price.
Whereas Williams headlines a deal along with one good prospect. It just becomes a question of what you'd rather give up, a major leaguer and two B prospects, or two A prospects and a B prospect, or even someone like Rupp.
If you just want to get a #4 type starter for depth and experience, different story, but those are easy to get.
And if you do trade Williams (or any of the major league OFs) there's a place for Quinn.