We opted out of the 'half-off' US domestic flight for our cruise. Price savings was minimal (if anything at all) for us. I like choosing the exact time and airline, minimizing stops, ...and using points to pay for it.
On the flip side, if itineraries change, its handy to have NCL handle those changes at no extra cost.
If a significant delay happens, we'll be on the hook for the cost (and/or insurance) involved. Admittedly, this is how most cruises w/o flights attached to them work anyway.