If you are printing a document in multiple copies and in two-sided mode and you don't check the collate control, you don't get what you expect.

Example: 2 pages doc, printed in 2 copies:
- without collate: you get 2 sheets, the first with page 1 on both sides, and the second with page two on both sides
- with collate: you get the right result: each sheet has page 1 on front side and page 2 on reverse side

In my opinion the collate check shouldn't refer to the two sides of a sheet in two-sided printing, it should only refer to the different sheets.

Example of correct behaviour: 4 pages doc printed in 2 copied:
- without collate: you should get 4 sheets:
-- sheet 1 and 2 with page 1 on front side and page 2 on reverse side
-- sheet 3 and 4 with page 3 on front side and page 4 on reverse side
- with collate: you should get 4 sheets.
-- sheet 1 and 3 with page 1 on front side and page 2 on reverse side
-- sheet 2 and 4 with page 3 on front side and page 4 on reverse side

Anyway, I think that it should be more user-friendly that the collate checkbox be enabled by default, because when you print a multi page doc you usually want to have each copy collated.