I thought we were only referring to the 1977 team. GB's OP stated that every Cowboy Super Bowl winner had a "bell cow" back, and in 1977, Dorsett was nowhere near the top 10 in rushing attempts . His 208 carries was less than 2.0 carries per game more than Robert Newhouse. 12.9 carries per game (Newhous) is more than just "some carries".
You are absolutely correct that it was a product of the Landry offense. Landry rarely ran out of an I-formation during Dorsett's early years. Therefore, Newhouse didn't see a lot of action at fullback during those years. Landry ran a split-back fpormation most of the time, so what you had was really two halfbacks, rather than a TB and FB. This changed, of course, by Dorsett's 4th year in the league, and I'm perfectly willing to characterize Dorsett as a "bell cow" after the first three years. I was simply taking issue with the notion that "every Super Bowl winner in Dallas' history" had a bell cow type of back.