If you need a fifth center back for one game, it's no big deal to spot-start Ring or one of the fullbacks there. If you need one for longer than that, chances are you'll want to go out and sign a starting-quality player instead of trusting the type of guy who'd fill a fifth CB slot. Look at it this way: Sands was our fifth CB before Hountondji's injury, and everybody was cool with that because he wasn't about play there. But lo and behold, as soon as it looks like Sands might actually get minutes at CB in some future contingency, people are clamoring to sign somebody else. So how much do you really need a fifth CB?
I agree that it's worth picking up a body for depth, since you never know what could happen and we don't have a reserve team to draw on in an emergency, but that's a whole lot lower on the list of priorities than a third CM, who's going to get regular minutes all season long.