First, this is the warlock forum, we don't cast chain lightning.
Second, Simcraft takes this stuff into account. It uses the same priority list we do, casts stuff spell by spell, checks what happens, then continues on the list. If simcraft misses 500 immolates in a row, it will cast 501 in a row, and record the whole thing. That is exactly why we go through 10k iterations when we get stat weights - it removes the randomness of RNG as much as possible.
No one is saying 100% hit is a bad thing. We ARE saying that 100% hit at all cost is a net DPS loss, and you're making the decision to lower your dps to make it more regular. Its like eating fiber: most of us aren't gonna get the hard poopies any time soon if we don't eat it, but the RNG of bowel movements may bite us in the ass sometime in the future.
Roughly speaking, if the socket bonus is at least +20 Int, then gem for the color of the socket, otherwise go straight Int.