Great topic!

For three seasons, I must admit that my "back-up" gear is minimal:

-I always take one mini-bic, a small box of strike anywhere matches and stormproof matches. I could do without the SAMs, but for 0.2oz, I don't worry too much.

-I take both Chlorine Tablets and the Aquamira Pro Filter. The latter is for emergency and weighs only 2oz.

-Clothing extras: my biggest thing is gloves-- I take Smartwool liners, BD Fleece gloves and rain mitts on pretty much every trip in the high country. Cold hands and poor dexterity is not fun.

In winter-- I also bring additional socks that count as back-up.

Beyond that, I don't bring too many extras.