The benefit of interactivity on the format cannot be found by merely analyzing the mechanics of the sequence, “player A casts spell”, “player B counters”.
The benefits come from what lies beneath the surface of that. Before playing that spell, player A must ask herself, “Does player B have a counterspell or not?” A’s optimal line might differ based on the answer. She must compute the probability of winning with line 1 if B has a counterspell vs if he does not, and similarly for line 2, and then combine the computations into a decision. This requires skill. Hence counterspells increase the skill level of the format. And that is a good thing.
The more dependent my optimal line is on information that is hidden from me - information which can be probabilistically deduced from data and logic - the higher the quality of gameplay.