If you're actually solo, the choice should be obvious. In small groups, the answer probably depends on what you're doing and your group composition. In a full fellowship, let alone a raid, the choice should be obvious again, the other direction.
So why do you want to lock it behind Red Line, then? Everything you've said is true for the other lines as well. The issue you are afraid of exists for a red captain too. Why not to make the stance for every spec? Or better, just increase the dps to the level we won't need to swap stances.
Let me eleborate on how it works in Everquest 2, for instance (not an ideal example, there are a lot of balance issues there, but still). While being solo a dps class does 100 dps, a support class and a tank do up to 70, a healer up to 50. But in the group they buff each other and so the result is 150 for a dps, 100 for a tank and a suport and 70 for a healer. In lotro non-dps classes do almost no dps, no matter solo or in the group.