Oh, I see what you’re saying. I misunderstood, I thought you were saying nothing stops you from doing the free OA and a regular OA.Which changes nothing. You can only make one OA per turn normally anyway. It doesn't prevent me from making an OA instead against a target I haven't marked (using my reaction, of course, and without advantage).
What Mark does is allow you a free OA with advantage with no cost at all to the PC or creature using it.
Yes, I agree with you that Marking just seems to give you free advantage OAs at no cost, which is why I wouldn’t use it without Facing. With Facing though, I feel that free OAs are warranted, to compensate for the additional competition for the reaction resource.
But again, I’ve taken up enough of this thread with discussion of Facing and Marking. I recommend trying them together, but if it isn’t for you, no worries!