Honestly never understood why an owner hadn't done this earlier. The jersey sales & buzz wouldn't seem to hurt a team's financials.
Roster spots are really valuable in the NFL though. You don't have many of them that you can mess around with. You only get 53 active roster spots and only a maximum of 48 can dress for a game. Teams usually only keep 3-4 tight ends in the roster to begin with and you might have 3 of them on the field at a time, depending on the situation. There's not a lot of room for gimmick players who can't play.
Tebow is shorter than average for the position, lighter than average, and at 4.7 in the 40 was just average speed for the position, and that's assuming he hasn't slowed down since his combine workout eleven years ago. And with tight ends, the slower guys tend to be the bigger ones who focus on blocking, while the smaller guys are the fast ones. Tebow is the inverse of that: a short, light, slow tight end. Hard to see where he fits.
If he can play, so be it. Good on him. But if you're only giving him a 53 man roster spot as a gimmick to sell jerseys, it is not a good move if you want to actually win football games.