I think the hallmark of a great leader is the ability to trust one's generals, so to speak. I know I'm speaking in vague terms here, but I think it's Roman's job to offer assessments of the various options, and the potential for each. Along with that would be his preference, but if his preference doesn't match up with the type of team Ryan is trying to build, I think it's Roman's obligation to adjust to Rex, not the other way around. If Roman truly believes that one option is going to result in fewer wins, then he should be making a strong case against it. But, if the conversation is about, "this is how we win with this guy, and this is how we win with the other guy..." then that is how Rex will build consensus.
I really like eball's hypothetical conversation in this regard, and the more I think about it (and, I am well aware that I am just pondering the ramifications of a thin rumor), the more I could understand how Roman would prefer Cassel over TT. By most accounts, Cassel reads the field better. That is no small thing, and I could see how that would give Roman more options in running, and controlling his offense. And, I could see how that would not necessarily mesh with what Rex is trying to build.