There's a real difference between a guy who kills the ball for his entire minor league career, reaches the majors, and still hits at a decent-but-not-great clip in his novice campaign, whose only real fault as a prospect is that he started his minor league career at the age of 25, and a guy who kicks around the minors forever and finally turns it on at age 32. One is just an unconventional prospect, the other is a bona fide fluke.
I mean we're talking about a guy with a career minor league line of .272/.333/.404 here. The chance of major regression is NOT trivial. Sure, Nava could bomb out too, but as he hasn't yet, he's a better longterm bet than McDonald. The only reason I make no fuss about the current roster is because we need a backup CF and can't leave that to Drew. Nava doesn't help us there.
As for Blanco, he's got 600 at bats in the big leagues, most as a starting CF, and while power will never be in his game, he's a good bet to be an outfield version of Scutaro, and has a career MLB OBP of about .350 with useful speed. No one would mistake him for a starter here, but as a speedy bench RHH with an OBP/speed focus, he's a useful player that doesn't belong in the minors.