To be perfectly honest, I feel that even if Smoltz doesn't pitch a single inning, that he still carries some value for the Sox. Sure it wouldn't be the wisest investment, but he knows how to pitch, and I think his veteran presence would do wonders for Buchholz, and it wouldn't hurt Lester, Bowden, Masterson and all the young guys (and Beckett and Dice-K can learn from Smoltz as well). Smoltz can help pitchers survive without their top-stuff, which would help in later years, but can also teach some of the players how to properly use their stuff when they have it.
I doubt Smoltz will be like John Farrell or anything, but I think the mere presence and knowledge bank that Smoltz has accumulated over the years is close to worth the 5 million alone, so if he can pitch a Smoltz like season or anything near average then all the better.
Smoltz also seems like a guy that would bring fans into seats, not that the Red Sox would have any problem with that. But given this economy that'll be more of a factor and that could help Smoltz pay for himself.