Red Sox rotation: Depth or surplus?

MLB Trade Rumors speculates about someone else's speculation (after which, I will do some speculating of my own) ...

    ESPN's Buster Olney speculates on Boston's pitching surplus:
    Eventually, it figures that Justin Masterson will go back to the Boston bullpen, and that will create the spot in the rotation for Daisuke Matsuzaka. If the Red Sox want to create another for Clay Buchholz, they would always have the option of taking offers for a veteran pitcher who has had quality starts in four of his six outings. That guy is Brad Penny, who might be a nice fit for a team like the Milwaukee Brewers or the Mets. That's all speculation at this point. Even if Buchholz can't supplant Penny, how about John Smoltz? He's back on track and may even pitch an extended spring training game on Friday. The surplus also includes 22 year-old Michael Bowden, he of the 1.01 ERA in five Triple A starts.

    The best bet for the Sox may be to just hoard this depth in the event of an injury. Even though most expect Jon Lester, Josh Beckett, and Penny to pitch much better from here on out, the fact is that Boston's rotation ranks dead last in the AL with a collective 5.81 ERA. They're 20-12 despite that stat.

Look, I'm all for rotation depth, if you can afford it. Obviously, so are the Red Sox (who can afford it). But one can take a good idea too far, and I'm not sure if even the Red Sox can afford the luxury of eight or nine viable starters. My guess is that eventually they'll decide to package some of that "surplus" and pick up a shortstop and perhaps a bit of outfield depth. And maybe more if David Ortiz doesn't find his power stroke (homer-wise this season, he's 0 for 116).
Lester and Beckett will pitch better. Or at the very least, they'll be given every chance to pitch better. If neither does, the Red Sox are in trouble regardless of what else they do. If one does and one doesn't, the Sox still don't need eight or nine starters; six or seven should be plenty.

Buster (not quoted above) is right, though: Ideally, Penny pitches reasonably well and the Red Sox can find a trade partner. Because Beckett, Lester and Matsuzaka are too good to trade, and Bowden, Buchholz and perhaps Masterson are too young and cheap to trade. Ideally, anyway. In retrospect, it might eventually seem that signing Smoltz and Penny was overkill. But the Red Sox like to enter each season with a great number of rotation options, and so far it's hard to argue with the results.