It isn't, because as posted above by both Bellhorn and myself, you can endure the growing pains of a hitting prospect as starters and still win ballgames when usually, if a pitching prospect is getting hammered, you are actively losing games.. It's a very apples-to-oranges comparison, and we are still advocating for (and assuming they will get) semi-platoon partners for these players. How do you platoon a young pitcher you had to throw into the fire because of injury?
As for your other point, it's possible to somewhat protect yourself against injury by maintaining as much pitching depth as possible. The Red Sox are a special case because they have so much talent but so many question marks in their rotation. So why would you do away with depth, that just a couple weeks ago you were so keen on protecting as well? That makes no sense.
The main issue here is the Dempster/Peavy debate. You could probably replace his (Dempster's) innings with a couple garbage bin pickups, but Peavy's a very good starter when healthy, even though he has his own question marks. Keeping the group six-deep plus the prospects is certainly better than any other option, regardless of how much rationalizing you do for my preference to starting JBJ/XB because WMB is certainly not a rookie anymore. You know what's easy to acquire during the season? A decent OF or IF. You know what isn't easy to acquire during the season? A decent SP.