I don't mind carrying 2 QB's on the roster as long as the backup is the real deal who you firmly believe can fill in for a month in case Romo suffers a 3-4 week injury (and I don't worry about "jinxes" and superstition, I worry about probabilities), so at least your team can tread water and it doesn't completely sink your season.
I don't feel confident Brad Johnson could fill in for a month without breaking one of his brittle old man bones or his noodle arm falling off. Then where does that leave us? Scouring the streets looking for someone to fill in for a few weeks, hoping he can pick up the offense in a few days. At least if you carry 3 QB's you have someone who has been all the way through mini camps and training camps and should at the very least know the offense.
Secondly, you should draft with the next 3-4 years in mind, because that's how long these rookies will be under contract. You don't draft just looking at next year. The Cowboys are going to need a long term solution at backup QB. Brad Johnson is in the last year of his contract. He certainly isn't a long term solution. Why not use just a small amount of foresight today and fill a hole cheaply that is going to be a pressing issue next offseason (and the offseason after that and the offseason after that). Just because there are always journeymen veteran QB's out there doesn't mean that's the best way to go. Anyone remember Paul Justin?