I think you extend SOME of the young guys. Guys that have high ceilings. I just don't think Rafaela is that guy. While the contract is "worth it" per FanGraphs' calculations, the Sox would most likely pay him less if they went year to year. The reason you extend guys is to give the players security, while giving yourself a slight bargain. That's not happening with Rafaela. With Bello, I think the price was a little high, but for a starting pitcher with Bello's ceiling, it's fine. If they extended Dobbins and Fitts, I'd be fine with it too because of how expensive pitchers are (they'd most likely be way cheaper than Bello).
Anthony and Mayer SHOULD be prime candidates for extensions. They are potentially future All Star guys. Same as Campbell. Those are the everyday guys that you try to lock up. I don't think you try to lock up future 8-9th hitters unless it's a discount and it wasn't here. You lock up the core of the team. I don't think Rafaela is that. It's not a knock on Rafaela either. He's a good player. He's just not a guy you're building a team around.