I have been to some of the most remote areas of the world to hunt, including the back country of Africa and Australia, which often means the space and weight limitations are severe. Taking two bows is often difficult to manage, so I normally only take one bow on those remote hunts. I prefer to have a longbow when I have only one bow along, because the limbs are thicker, less subject to damage from striking an object or twisting a limb, and easier to string or unstring without a stringer. I have scratched, dinged and dented longbows before, but I have never broken one so it could not be used.
However, I ALWAYS take extra strings. As pointed out by others, strings can fail from wear, abrasion, cuts, and unknown reasons. They are inexpensive, compact and very light. Why would anyone NOT have a spare string handy on a hunt? I agree that a spare string is best when completely set up with the same silencers and nock point height as the primary string, but I'm not one to get too excited about exactly how well it is shot in. Just make sure you have a way to accurately measure brace height and to string and unstring the bow so you can make brace height adjustments, and you will be fine. I like to have a bow square and stringer in camp, but my blunt tipped practice arrow with length marks on the shaft carried in my quiver will work for brace height, and I can string my longbow without a stringer if I have to, so I can twist my string to keep the brace height right. Besides, a bow string can be used for many other purposes in a pinch.
Allan