I think it is worth taking on the paper, if you possibly can, since every paper will count when it comes to postdoc or academic job hunting. And once the thesis is done, it maybe be harder to get opportunities like this.
My humanities thesis has taken, so far, 3 years and 8 months. Yes, finishing in 3 years would have been great. But as it stands I will have 3 papers and 1 book chapter (which I am doing the torturous corrections for now, distracting me from thesis!) which is supposedly going to help me on the job market (it hasn't so far). Eitherway, meh - get as much as you can into print without going insane.
PS: I have gone insane.