If you already have Marathon Plus tires, you might not need a spare. I carried a spare Marathon Plus tire for a couple thousand miles through Canada and Alaska. It arrived folded in thirds from wallbike.com and I left it that way, zip tied to the back of my rear rack. After I got home that spare sat around for another year and a half. I recently got a very slow leak in my rear tire after about 6000 flat free miles. Only then did I finally use that spare tire to replace my rear Marathon Plus. My main motivation for replacing the old tire was because the tread was worn flat in the middle, I wasn't too concerned about flats occuring more frequently.

I know tires can get cut beyond repair. So you just have to weigh the odds of that happening (and having to delay your trip to get a new tire) against carrying a spare tire you may never need. How far you'll be riding and in what terrain/environment will obviously affect this decision.