If he's only a receiver, the team is easier to defend

You don't want to take away his pass catching skills or opportunities. You do want him to be an effective blocker so that when you aren't running a pass play he's more than just a decoy, but someone they need to account for as a blocker. Then when you do send him out on a pass play, he's more effective because they had to account for him as a blocker, so they couldn't just try and play a physical safety on him. You want the other team to have to make a bad choice and if he's an above average blocker, there is no good choice for them.