Hi Jacobfox

There's an old joke that to a small boy with a hammer, everything looks like a nail. To a guy that works for a pool plastering company, EVERY pool looks like it needs a plastering job.
Ben is far more knowledgeable than me, but it should be clear that if a crack is simply a crack on the inside of the pool, patching it should be sufficient. If it penetrates the outside of the pool, water MAY seep in--if you have ground water, and you'll have to fix it. If not, I'd guess it won't and perhaps just patching from the inside, such as with pool epoxy may be enough.

Carl