We don't know how the coaches exactly view how he fits.
I do agree with you that if he was only thought of as a WR then there'd be other guys I want before him.
Greathouse is intriguing because of his potential versatility in the offense. I can easily see him being a swiss army knife that Sark can use across multiple spots to put the defense in conflict.
If he can pull it off that level of versatility is hard to find.