Starks DID return when he was capable of playing. Why would we be thinking about what the season would have looked like if he didn't?
Even if Harrison thing was relevant to the discussion, cutting him when we did was a good decision. He couldn't stay with a team because he was not coachable (he has said this himself). Perhaps cutting him helped him realize that he couldn't get by on talent alone. After he got that feedback from us and from the Ravens, maybe Harrison learned from the experience. If your argument is that the Steelers and the Ravens can't evaluate LB talent, I don't know who you think would be qualified to do so.
I think that cutting Starks made him realise that he had to do more than show up to be good. I haven't seen him admit this publicly, but it seems like he was working much harder after he was cut than he is generally known for doing in the off-season. Maybe the fact that no one else in the league thought he was good enough made him work his ass off to get in game shape. If so, that's good motivation by Tomlin and cost savings to boot. I guess there was risk that he would get signed by someone else (just as there would be risk that he wouldn't get back into game shape if we kept him). We took that risk and it worked out because we signed him back (I haven't checked, but I would imagine that he was resigned at a lower number than he was cut at).
At the worst, this decision cost us one game in a season that we went 12-4. If that's the "worst personnel decision under Tomlin", I guess it's another way we can show that we're incredibly spoiled as Steelers fans.