If you wager on a game almost solely on the pitcher, then you better wager on the first 5 innings. I can't stress any harder. Bullpens are not reliable enough to handicap
I've heard this argument and I see where people are coming from.
At the same time, you have another situation where something is already built into the line.
On the contrary, I think bullpen fatigue is heavily overlooked. If you dig into every team to rate their bullpens, then keep track of who will likely not be available that day, you'll be able to find some teams that are going to be at a real disadvantage in the later innings. This will be especially true if you have a starter who generally doesn't work very deep into games.
I think this is angle in largely ignored, especially considering overnights are put out before the games are even played on that day.