The imprecision of short-term WAR numbers is one thing; the market value of a win is another. Just because we may not be able to say with much confidence whether a player did or didn't earn his money in any given year, that doesn't mean we don't have a pretty solid idea of what it would take, in terms of on-field value, to do that. The market value of a win is an empirical thing. Yes, it's in constant flux, and people using different methods to calculate it arrive at slightly different numbers, but that doesn't mean it's a matter of speculation.
The current market value of a win is roughly $7M. That means that at $10.5M a year, we are paying Rusney to be a 1.5-win, i.e., slightly below-average player. Does that mean the Sox would be satisfied with a below-average player in their lineup? Of course not. The point is just that $10.5M isn't a lot of money anymore. For a full-time position player acquired as a free agent, it's cheap. If you have an average player in your lineup, and you're paying him that, you're getting more than your money's worth. The Sox are clearly hoping Rusney can be an average-or-better player. If he is, they've filled a roster spot for several years at a nice discount. If not, they didn't risk a huge amount of money, and unless he's really awful, they may be able to move at least some of the contract.
So, to your point about "as long as he posts a defensive heavy 1 WAR we did OK with our money?" I think we're all on board now with the reality that WAR isn't precise enough to say that just because a guy "posts 1 WAR", we can assume that he was really a 1-win player. But if he really was a 1-win player--or more accurately, a 1.5-win player--then yes, we did OK with our money. That doesn't mean we did OK with our roster construction. If we have a 1.5-win player manning a full-time outfield spot, we have a problem. But at $10.5M a year, the problem is not that we're overpaying him.