I'm rarely affected by this rule, but I can see why this will make traveling some itineraries on DL absolutely impossible (such as the MSQ-FRA-US itinerary Xeno is describing).
Given DL's desire to monetize high-value service, I'm really surprised they can't find a way to make this work with a fee, rather than an absolute prohibition. Or to make it available for HVCs or Diamonds or BE travelers, or some other narrowed group.