I think the more important discussion is, what is most likely to fail, and what is required to remedy those failures. What is the risk of having an unfixable failure? Are you willing to manually inflate for a dive or two if you have a leaky inflator? Do you know your SAC rate well enough to do a shallow reef dive without an SPG? What compromises are you willing to make with your diving? If you are unwilling to make any compromises, your kit should cover more contingencies. If you're willing, you could dive with a single second stage, no SPG, and no inflator, and still have perfectly acceptable dives.
That being said, you can hedge your bets with a couple small bits and pieces that may minimize these compromises.
HP hoses leak from the spool often enough that an extra spool is worth carrying, and weighs virtually nothing. An extra reg set might sometimes be a tough sell. A $14 inflator and a zip tie is more convenient than an LPI rebuild kit and tools, more expensive, but not necessarily any real weight penalty and any monkey can swap one out, but definitely takes up more space. An extra HP seat for a quick swap will often get a reg through a vacation until you can get it home and do a full rebuild, again, small and virtually no weight penalty, but you'll need to be able to take the reg apart. A little bungee and some zip ties can go a long way.
For recreational diving in most places, if your kit takes up more space than half a mask box, you've got too much stuff. Depending on what compromises you're willing to make, you could fit you whole kit in an empty cigarette pack. It's all choices based on judgement. If I'm tropical recreational diving, I take virtually nothing. If I'm CCR cave diving, I've got half a tool box.