A bit of a bump, but I think it's worth it: while studying part of the caravan code in version 0.23.130.23a (i.e. the old 2D version), I located the code responsible for this behavior. Better yet, I looked in the same place in version 0.34.11 and it's still there (and with one extra bit).
Here is exactly how "emergency supplies" appear to work:
1. The caravan will try to bring enough meat/fish so that you have 5 units of food for each "carnivore" in your fortress. With stock raws, this does nothing at all (because dwarves are omnivores).
2. The caravan will try to bring enough assorted food (meat, fish, cheese, plants, and possibly others) so that you have 5 units of food for each citizen of your fortress.
3. The caravan will try to bring enough cloth and leather so that you have enough to have 5 pristine sets of clothes (1 shirt/cloak, 1 trousers/loincloth, and 1 shoe) for each citizen of your fortress.
4. The caravan will try to bring enough wood logs so that you have 1 log per citizen of your fortress. This didn't happen in 23a since caravans didn't bring wood logs back then.
All items in your fortress which are forbidden, rotten, or trader-owned are ignored.
For cases #1 and #2, it actually just uses the numbers displayed on your Z-Status screen - meat+fish for #1, and meat+fish+plant+other for #2 (yes, this includes fat and tallow, even though you can't eat them directly).
For cases #3 and #4, items which have wear ("x", "X", or "XX") are excluded.
For case #4, items which are considered to be armor (having a nonzero ARMORLEVEL) are ignored. The actual formula for the amount of cloth/leather to bring is "(population * 5) - (num_cloth + num_leather + min(num_armor, num_pants, num_shoes))", so keeping a large amount of stockpiled cloth and leather will stop caravans from bringing more. Interestingly, it doesn't seem to check clothing size, so having a bunch of stockpiled goblinite back in 40d was sufficient to stop this behavior (in 0.31 and later, only stuff from Humans, Kobolds, and Trolls will trigger this oddity).
Thus, the real reason why Elves (and all other caravans) bring so much cloth with their caravans is that they don't want your dwarves to be naked. Back when this behavior was first added, Dwarves got unhappy when their clothes wore out and even more unhappy when they were naked; by 40d all of the unhappy thoughts related to clothing were gone, but since version 0.34.06 brought them back, this behavior is much more useful than it used to be.