If Aceves goes into the rotation, I think that would weaken the pen more than it would strengthen the rotation. Doubront is a long shot, but the best shot on the list. Other than Padilla, the rest of that list has virtually no chance of contributing anything of significance in my opinion. You mention Julian Tavares as an example, but by the time Taveras was pressed into duty in 2007, he had spent 14 seasons in the majors and he had been a starting pitcher for 3 seasons for some crummy teams and he did show some ability to eat innings and win as many games as he lost. He spent most of his career as a reliever and was good enough to stay in the majors for 17 seasons and stay healthy. Silva and Maine are bums. Cook is injured beyond repair. He has nothing left. Miller is a bust. Padilla is the only shot. He's the only one of the sorry bunch that might do something, but he hasn't been healthy in 3 years.
You are still not saying how the much the decrease in the quality of our bullpen (resulting from moving Bard) will offset the improvement Bard brings to the rotation. I don't think Melancon will be close to the weapon in the 8th inning that Bard had become. It really comes down to the delta between the improvement to the rotation brought by Bard and the decline experienced by the bullpen by losing Bard. I just don't see that as being significant.
Finally, I think the addition of a starter easily could be the difference between a 95-96 win team and a 90-91 win team that misses the playoffs.