Yup. Interference with stadium ads.
He is a good blocker and a below average receiver. An actual receiving threat, albeit small one. Unless you are on the one yard line, it is much better to have him in than a third tackle. Even in that situation, giving a route to a guy who just blocks 85% of the time is less predictable than the guy who just essentially said "Nothing to see here."
A third tackle (not a starter), is likely to be an average to good blocker at best with no receiving ability to speak of at all. The drop off in skill from one area to the other (blocking vs receiving) is, in most all cases, likely larger in the receiving area.
If playing 3 tackles was a better option 100% of the time, a coach might have tried it at least once. Maybe it happened a long time ago, but I can think of no instance of this in recent memory.