I totally disagree. Salty's defense is a bit lacking, but it's passable. In general, you don't lose games because your catcher can't throw out 40% of would-be base stealers. Catcher is a weak position defensively throughout baseball, its what makes it such a premium position. There are a very select few who are good hitters and good defenders, it's usually one of the other. Salty is a somewhere in between, he calls a good game, is below average defensively, and above average offensively.
I think Salty is a great fit in the Red Sox clubhouse, and from what I can tell he handles the pitching staff well. Don't forget how onerous catcher transitions can be, it took Salty a full year to call a good game and for the pitchers to get used to him.
Furthermore, Salty has the 4th best OPS in baseball of catchers with at least 300 ABs at .779. He's not elite by any standards, but he's above average for a catcher. He's 4th in doubles at his position, and 6th in home runs. So I don't know how you can say he can't "hit or hit for power". By any measure he is one the better hitting catchers and has been for the last few years.
I've gotten used to Salty, his pace and his defense were tough to take at first, but as the pitchers adapted to him I became more comfortable with him too. I'd be pleased if the Sox sign him for 3-4 years. We know he can handle Boston which is another good aspect of Salty's character and an aspect of his game that should not be overlooked.
Somebody like McCann may appear to be a better option but can he handle the pressure cooker environment that comes with signing a rich deal and playing in Boston? Can he handle the AL? The AL East?