Where Romo was drafted has no bearing on the premise. The round a player is picked in, at RB, is indicative of what kind of physical specijmen they are. You're comparing a heavily mental position to a mostly physical one. That's ridiculous.
As for bad or unproductive QBs: Christian Ponder, Gus Frerotte, Tavaris Jackson and Matt Cassell for Peterson. Dieter Brock, Jeff Kemp, washed up Steve Bartkowski/rookie Jim Everett for Eric Dickerson. Scott Mitchell for Barry Sanders. Dan Pastorini and horribly washed up Ken Stabler for Earl Campbell. Doug Flutie and pre-2004 Drew Brees for Tomlinson. Bob Avellini, Mike Phipps and Vince Evans for Walter Payton. Jay Fiedler for Ricky Williams.
All of those guys were able to have productive seasons in years where the passing game was way below average. I don't think it's unfair to say that Murray owes some of his success to the league leader in QB rating keeping the defense honest.