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I think you'd use commas in amounts but not years.
In currency it makes it easier to read, for example (for smaller amounts I consider it optional). You can see how much £2000 is at a glance, but £12,000 benefits from the comma, as does $2,000,000. Quantity or weight usually has larger units you can use in preference to larger numbers.
Calendar years are only ever four digits (for the foreseeable future at least) and context makes it unnecessary and superfluous to add a comma. That's what I think anyway.
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