I think that fighting the hypothetical is supposed to prevent these scenarios:

A: Proposes a strategy S.
B: "How well would strategy S work in an alternative universe, completely different from ours?"
A: "Not well."
B: "Therefore, let's not use S in our universe. (Let's use my favorite strategy S2 instead. It works better in the alternative universe.)"

Now if A starts objecting after the fourth line, B can say: "Hey, you already admitted that strategy S doesn't work, didn't you?" So it is better from A to say this already after the second line.

Remember that the goal of the debate is usually to convince a third observer, and A wants to prevent them from being primed by the conclusion in the fourth line.