The problem for me isn't so much that you have to do it to complete the quest, it's the execution of you having to do it. You can't voice any protests about it to Karliah to have her explain that you really do have to go through with this or that the Guild's always had three of its top members as Nightingales. It just seems a bit rushed and vague not out of an organic way of fleshing out the story, but just out rushing it.
Also, the problem with your argument is that if I walk in on a room with my friends doing drugs, and I don't want to join them in doing drugs, my leaving doesn't make my friends sit there forever waiting for me to show up again.
Also, i would like to add on. There should be another option besides becoming a nightingale. So you are the dragonborn, you can take down mercer. You have akatosh and (prolly other daedras) on your back.
Unless Karliah says 'You cannot kill mercer because he is invulnerable, since he is a nightingale and you are not. Only way to kill him is to become a nightingale'.
That, if she did say that, i can accept. But she didnt say it, so i m assuming that Mercer can be killed even though you aint a nightingale. Dont get me wrong, i love the armor and skills, but nocturnal is a ****