Ausmus is one of the best defensive catchers in the league. The Astros typically have enough offense elsewhere to make carrying his bat a non-issue. See also Everett, Adam.
It doesn't work like that. You have to judge each player on their own merits. A player's bat isn't more or less valuable because of their team.
Let's say Adam Everett is worth about 15 runs above average defensively. Does that sound good? This year he's already 8 runs below average offensively.
Ausmus for his Houston career has been 83 runs below the average catcher on offense. Do you think it's possible that he was 83 runs above average on his defense?