It's fine to say that Navarro is an offensive catcher, but it takes it way too far to say that he can cover DH. He projects to be a below average MLB hitter (and he has been his entire time in Toronto).
The merits of Dioner Navarro depend entirely on whether or not you buy in to the newfangled advanced catcher stats. If you do entirely, then he is essentially a replacement level player. Maybe you can add some value for "game-calling", which escapes numerical evaluation so far, but that's speculative.
If you do not buy into the new stats re: framing and blocking, then Navarro is a solid backup.
For reference: His 2.1 fangraphs WAR in 2014 becomes 0.6 when we include framing numbers. His 0.5 WAR from last year becomes 0.2.