I don't see how getting into a bidding war with the Yankees and Mariners over Ellsbury (who averaged only 113 games a season in his prime) should be considered "a fairly easy way to insulate themselves," ( from injury to CF). Committing $154 mil seems kind of hard to me. And they did offer Drew $14.1/1yr, which was "fairly easy," but committing to a multi-year deal was much harder, particularly since it would have made it impossible to gauge the MLB-worthiness of Middlebrooks (assuming Bogaerts went to third.) So it would have cost not only the added money to Drew, it also would have been losing any potential value in Middlebrooks. If WMB can be a 2.5 - 3.0 WAR player, at minimum salary, then that's another $15 million lost in opportunity cost.
No team can afford to have the depth you want at every position. Not even the Yankees. They're spending over $220 mil on payroll this year, and their backup first baseman is their starting 3rd baseman. And their starting 1st baseman is recovering from wrist surgery and has a history of hamstring problems. If the Yankees can't afford multiple contingencies at every position, then how can any other franchise do it. Sizemore (either at CF or in RF with Victorino in CF) may not be the ideal solution to depth, but it's hardly a train-wreck. And if the train does go off the rails, they have trade bait to address it.