I'm trying to compile a fairly comprehensive list of the uses of the ablative, because I like lists and I like learning from lists, and I ran into this thing--the ablative of association. Thing is, it's not in Wheelock's, nor in the other textbook that I'm referring to (D'Ooge); it only shows up, as far as I can Google, in Bennett's New Latin Grammar (which is not exactly new any more). Which piqued my curiosity: is this just a peculiarity of Bennett? Has this use of the ablative been subsumed in later classifications of the uses? Or is he being ultra-thorough in his listing? Hard to imagine Wheelock's missing much, though.
For reference, this is the NLG's explanation:
"The Ablative is often used with verbs of joining, mixing, clinging, exchanging; also with assuēscō, cōnsuēscō, assuēfaciō, and some others to denote association; as,—
improbitās scelere jūncta, badness joined with crime; āēr calōre admixtus, air mixed with heat; assuētus labōre, accustomed to (lit. familiarized with) toil; pācem bellō permūtant, they change peace for (lit. with) war."