He may not have been drafted to be a blocker but the fact that he can't has undoubtedly relegated him to a handful of snaps per game. Whether or not these receiving TEs block for other teams is largely irrelevant because they need to be able to block for Dallas because Dallas asks them to.
The fact that they are asked to block is probably the reason people are upset that they can't block. They are asked to block but can't. Hannah has 205 snaps from PFF, only 69 of which have come from running routes. If you combine Hannah and Escobar, even with Escobar basically running twice as many routes as he has blocks, the two of them are still blocking on about 55% of the plays. Because they cannot block, any time they aren't running routes they're a liability and likely contributing to this awful running game.
How can you say people don't understand and then follow it up by saying that their blocking doesn't matter when, A) they are asked to block more than they are allowed to run routes, and B) blocking is the sole reason why Escobar hasn't really been on the field? Clearly blocking is important for Dallas.
Going a step further, it's important for all of these hybrid TE because without blocking they'd just be WRs.
Jimmy Graham still blocks 30% of the time. Julius Thomas and Vernon Davis block more than they run routes, 45.8% and 48.6% respectively. Gronkowski blocks about 35% of the time which is the same for Tony Gonzalez. Jordan Cameron blocks a little more at about 38% of the time.
You are right by suggesting that the hybrid TEs are primarily used to catch passes. That doesn't change anything regarding their need to have blocking ability. Even though Escobar wasn't drafted with blocking in mind, he absolutely has to be able to block. First and foremost, the team is going to ask him to block so that in itself is reason enough.
More importantly, if he cannot block it limits the ability for him to create mismatches. If he never blocks, he's a WR and a slow one at that. Teams will play coverage all day on a guy who runs a 4.8 40 yard dash because you aren't going to get beat deep and even their worst coverage corner has the speed to make up ground and try to get a hand on the ball.