I don't view Kuroda or Jackson or even an old injury prone Oswalt as top tier pitchers. They are IMO most definitely in the second tier, and we can't afford them. Once you dip into the third tier, you don't know what you are getting. They are just warm bodies who, if you are very lucky, can take the ball every 5th day. They are usually not good at all. Maholm is a warm body with a live arm, but he is not good.
With regard to your optimistic outlook about Bard, we're all hoping that he transitions well to starting, but the truth is no one, not even Sox management, knows how this will turn out. He could be very solid. He could blow up. This could ruin his career like it did to Joba. We just don't know. We also don't know about Aceves. The odds are against both adapting well, pitching effectively and being durable enough to give us 175-200 innings. They need one more solid starter so they don't have to roll the dice on 2 relievers making the transition to starting. I don't think Maholm is the answer.