Since Perkins has a long-term deal, you have to consider how good he plays over the lifetime of the deal and not just now. Can you honestly say it would be shocking if Stiemsma turned out to be clearly the better value at center two years from now?
If The Steimstress is as good in two years as Perkins is today, someone's going to offer him a deal something like... oh, four years, $30M. Big men ALWAYS get overpaid, people seem to want to forget that there's always a team that will overpay for any talent at the 5 spot. As we (more than any other team) is so well-aware, it's not that easy to get a legit big man.
I get that Perkins is an imperfect player. I get (kinda) Danny's decision to trade him for (perceived) value, since he didn't want a Perk extension to impinge on his cap space this summer (not that the cap space is likely to do us any good).
What I don't get is what seems to be this irrational hatred of Perk. The only people that ever seem to complain about what Perk brings to the table, or how much he's paid, are Celtics fans.
Steimer blocks shots; that's about it. That's nice, and you see signs that maybe he can develop into something else, but he's pretty much the polar opposite of Perk in most regards. He's certainly isn't a guy that you ever expect will anchor a top-tier defense.
EDIT: I did this last summer, I don't feel like doing it again. Take whatever list of "good," starting-caliber NBA centers you want, and look at how much people are getting paid. Except for guys on their rookie deals, you don't see too many players earning materially less than Perk. $8M-$9M/year is pretty much the minimum going rate for a starting 5.