Why do some disk brake pads use split-pins or bolts to retain metallic or resin pads? With Shimano disc brakes, there are two options for types of brake pads: resin or metallic. My understanding is that the metallic brakes generally have more stopping power while resin are more modulated and squeal less. 

With that said, as far as I have seen, some use a split-pin to stay in place, while others use a bolt.


A split pin used on both organic-resin and sintered-metallic pads

This is strange to me: I feel less comfortable knowing my ability to stop is based on a bent split-pin rather than a screwed-in bolt! Is there some explanation for this? 

My best guess is that the metallic pads create more heat, which could warp threads, but that's just a guess.
Some caliper models use the threaded style pin and some use the bent cotter pin type one. Several of the pad shapes are used in calipers of either type. Every Shimano replacement pad that is compatible with a caliper that uses the bent pin type retention pins will come with any replacement pin or pins that may be needed, plural because there are actually 3 length/profile variations of bent pin across Shimano calipers. There is no metallic versus organic distinction involved. 

For the most part the calipers that have the threads are the more expensive ones, or at least that's how they've been doing it recently. To date I believe all the road calipers are threaded, but that's subject to change. On all these calipers the pin is considered part of the caliper and is re-used. If they're lost or damaged, a specific replacement has to be obtained; there is some overlap but it's not dimensioned the same across all the Shimano calipers that use threaded retention pins. If the threaded pin is lost or damaged, a bent cotter type one of appropriate length can be substituted without issue.

Neither style is subjected to braking loads in any way. The pin is a loose fit in the hole of the pads. Braking loads from the pad are entirely pushing on the lips of the caliper that the pads rest against. The pins are there solely to prevent the pads from falling out when you're not braking.