It really doesn't. There aren't a ton of those guys, either 25 in 2014 and a meaningful percentage of the pitchers who throw that poorly but are allowed to accrue that kind of innings are ex-aces, guys like Verlander, Buchholz, Wilson, Haren, Miley, and Lincecum. Lackey's terrible 2011 with the Sox is a memorable example. You only get the rope to throw that badly (6.41 ERA; 4/71 FIP) for that many innings (160) if you have demonstrable ace upside on your CV and, generally, the contract to match. A few more are young guys with legitimate upside: Bauer, Odorizzi guys like that. Again, there's enough upside and few enough alternatives that they get 30 opportunities to be loudly mediocre for five plus innings.
The guy you basically mean is Kyle Kendrick: a pitcher without much upside whose only real value consists in eating innings. There are like five of these guys in the game right now: Kendrick, Hutchison, Guthrie, Danks, and maybe Nuno or Roberto Hernandez?
So yes, the Royals would gladly trade a good relief pitcher for an SP who could be reliably projected to approach 200 IP, even without a shiny ERA. But that's not a number 5, and that's not really what the Sox have to trade. Their needs and ours don't actually correspond well. They're not going to trade 60 excellent high-leverage IP from a great reliever for 115 IP worth of mediocre starts, which is basically what the young guys can be projected for. (That's the Steamer projection for Workman.)
These guys are young: one of them will likely step up and seize a rotation spot by performing better than that, but KC would have to be confident they could predict which to send Wade Davis back to us.