With the bat, Nieves is probably worse than most of the pitchers on the team -- his career .227/.273/.297 line is flat out bad, even by backup catcher standards. If he's bad with the bat, he must at least be good behind the plate, right?
Not really. In nearly 1800 career innings, Nieves has only caught 24% of opposing basestealers.
The good news is that Nieves is so bad, that even if he does win the backup catching job this spring, at least Ron Roenicke won't be tempted to give him significant playing time over Jonathan Lucroy. I don't know why the team would give him a major league contract instead of a minor league deal with an invite to spring training, but in the end, it's a move that probably won't have too many negative consequences. There's no real roster crunch yet, he likely won't be making much more than the league minimum, and there's been nothing to indicate that this is a multiyear deal. If he doesn't work out this spring, they can cut him loose without losing too much. This is mostly a harmless move, even if it does make George Kottaras sweat a bit.