Because having a sixth OL is only good in running plays or if we want to play action deep on an early down.
What do we do on 3rd and 6 when it is too far to run the ball and we cannot pass protect? Having a sixth OL accomplishes nothing. At least if you have a TE who can walk and chew gum at the same time, he can chip the defensive end, buy Hunter/Sanchez and extra half second, and then become a dump-off option.
We had the option to get someone cheap like Daniel Fells or Daniel Graham, in fact Graham was available all summer until a week ago, but we chose not to. We need a complement to Keller, especially if we want to live up to this ground and pound we are preaching, but as it stands our two backup TEs are crappier replicas of Keller.
3rd and 6 we're not putting in a blocking TE. Hartstock or Mulligan weren't playing when it was 3rd and 6. They were absolute luggage on passing plays.
In that situation, put in Baker/Cumberland and let them attempt the chip. Its better than having Ben Hartstock run 3 yards and fall down.