My riding nowadays is almost exclusively long day rides. I always carry a chain tool but not a spare link. My logic is that if I have to remove one link I can complete the ride and replace the link later. I've always imagined that the link most likely to fail is the one I've most recently used to join the ends when I last refitted the chain. When there is such a simple fix to a problem which would otherwise be a ride terminator I can't see any reason not to carry the tool.
As others have said, sorting out a stiff link by the road needs a chain tool, and I'm sure I've done that a few times over the years.
Someone commented above that a small hand tool is too difficult to operate. Until very recently I've been carrying a bad one - small and light but very hard to use. For a while I assumed it was me, but in the end I reasoned that the screw threads must be ill-fitting. When it reached the point where maximum pressure is applied, it needed too much strength simply to turn the screw, making it almost impossible to do the job in hand. I've replaced it with another, which works much more easily.