That's simply not true - I have had open-dated ticket coupons with "required transits" involved on a number of occasions (mainly because of the 2 stopovers in continent of origin rule, but the principle's the same). I also quite happily got off a LHR-MNL flight at HKG (in the days when BA did that route) and never reboarded the last "sub-sector" to MNL (which was the termination point for the ticket) without any penalty, and have failed to take the last full segment on a number of occasions for various reasons. Again, no problem.

Note though that the rules do now penalise "no shows", so to be safe it would be better to have the segment(s) open-dated.