First of all, Christians don't establish doctrine on one single mention of something in the bible. You can have an opinion, you can believe it, but it's not good doctrine. For instance, I believe in a pre-tribulation rapture but I won't die on that hill. And I won't reject someone who is a mid- or post tribber.
Secondly, Paul says THEY are baptising for the dead. Not we are . Google for a site that translates the original Greek into English. Fascinating. I wish I knew Greek.
Third, the subject being discussed was the resurrection. Apparently those who were batising for the dead didn't believe in a resurrection. Paul was asking what therefore the point was.
Mind bending/boggling placemarker