I'm a vampire...so..techniqualy do i even have a soul? (havnt read anything about that in TES lore)
anyway if your character doesnt fit the oath dont take it you dont have to, for one my character is a Shadowcale Argonian vampire so personally his mission (as well as being close to Sithis) he wants to honor the Night Mother(sithis?) who started off in the thieves guild leaving a calling card of a white and black stone in the eye sockets of the people he/she robbed and thus spreading fear through the world while also being a DB agent in either sense Nocturnal is one of the night ''essences'' in my view and her luck and gifts will help hone his skills for Sithis who he is ultimatley tied too.
.....go figure.....gives me a nice story/character to play on in thieves/DB guilds

i rambled....... bottom line dont want it? dont take it.
It bothers me that you aren't given any choice in the matter. In my opinion there should have been at least 2 choices, perhaps 3.
So, when Karliah says something like: "We're going to make the pact with Nocturnal, are you ready?" I would have liked the conversation options to be:
1) "Yes, I'm ready."
2) "Why don't you and Brynjolf make the pact? I'd rather not." (This way you just don't get the special powers and armor. If a third person is necessary for the pact, someone else is drafted in from among the other thieves guild members or something).
3) "Screw this. I'm not going through all this to catch Mercer." (You fail the quest and get kicked out the Thieves Guild).
This questline isn't the only one where you're given little choice. I can think of others. It's either lazy writing or because Bethesda wanted to have a linear path that didn't confuse the COD players.
This is only my opinion - I'm sure a lot of people wouldn't see the point, but as a long-time roleplayer I think it should have offered multiple paths.
well i know exactly what you mean and i Totally agree that way you could advance in the quesline while not being sworn to Nocturnal (mercer could always have increased attack/HP to highlight that her ''luck'' is not with you.)
anyway besides that Karliah does say ''were going to go and make the Oath, are you ready?'' you can choose ''no not yet'' and leave it there until:
A) your character has a good reason/excuse

you just want no part of it.
but as Korr says they stand there forever and it is so incredible anoying knowing its ''unfinsihed'' like there waiting in limbo.