First off, I don't accept the premise that there is such a thing as a "number 5 starter."
Each starter is as important as the other 4. The games they start count the same.
The goal isn't to have 1 guy way above average, 1 guy a little above average, one guy average, one guy a little below average, and one way below average (a "1", a "2", a "3" a "4", and a "5".)
The goal is to have 5 guys all way above average. Come as close to that as you can.
Planning for a "5" in your rotation is stupid. It's like planning for 2 guys that can't hit, because you have 2 guys who can.
And, in the specific case of Randy Dobnak, he's awful.