Use of "Or", inclusive or exclusive?

I agree with you (and have written to this effect here) that English or is ambiguous.

Alongside your “she couldn’t read or write” and Andrew Lazarus’ “You must be crazy or stupid” there are ordinary constructions like “Help yourself to chicken or ribs or chili or whatever strikes your fancy” and “I like Dickens better than Trollope or Scott or Thackeray” in which or is clearly not exclusive. If it were not so, lawyers and technical writers would never trouble to insert “but not both” to specifically exclude an inclusive reading, just as they insert “or both” to specifically exclude an exclusive reading.

Under ordinary circumstances, the context makes clear which meaning is intended. If a waiter tells you you may have mashed potatoes or fries with your steak, both of you know perfectly well he means one or the other but not both. On the other hand, if he asks if you would like coffee or dessert you do not understand him to forbid your ordering both coffee and a slice of pie.

But when there is no such context I would follow tchrist this far: the default reading is exclusive.

It is untrue that the natural English or is clearly and uniformly disjunctive. If it were, it would be impossible to add "or both" or "or whatever" to an or clause, or to make a list of more than two items, as in StoneyB's examples:

“Help yourself to chicken or ribs or chili or whatever strikes your fancy” and “I like Dickens better than Trollope or Scott or Thackeray”

RJB is correct that

how a person uses 'or' is very much defined by their perception of what they mean, rather than by any formal rules of the English language

Often we use or to convey a feeling of subjective doubt:

He speaks French, Spanish, Portuguese and German or Finnish, I forget...

The point is not "either G/or F"; the point is "maybe G, maybe F...". And while we tend to use "either.. or..." to strengthen the sense of "not and", this is a matter of degree not kind. There's a nice page on contract drafting here and StoneyB's superb coffee & cake example makes it clear that context is key, (or) perhaps even king. Note also that English uses or in negative sentences such as the following:

Bob's not in his room or his study.

Clearly in logical terms what is meant here is "Bob is not in his room and Bob is not in his study". Natural English does not use and because the conjunction falls within the scope of the negation (and it's obvious that Bob can't be in both places at once, so why say it?). In more formal English, we would say he is neither in his room nor in his study. Some languages, such as Chinese, use a positive conjunction and two negative particles to convey this idea (他不在房間也不在書房).

But I'd agree that, apart from contextual considerations, the default meaning of or does tend to be disjunctive. Frequently when translating the Chinese or, I have to use and in English because the or is insufficiently inclusive.