I think you might be quoting a very unusual situation. In my experiences and my colleagues experiences both in military diving and commercial diving with hyperbaric services - The majority of people have a reaction at 3.2 - even in chamber treatments you're comfortably breathing pure oxygen at 18m (2.8 ppo2) However there are always CNS exciters like cold for example that can cause vascular constriction. In my professional opinion, many "Nitrox" divers experience narcosis before cns toxicty and relate it to the nitrox rather then the nitrogen.
The 1.4 - 1.6 rule is extremely conservative. My time in commercial diving had me breathing pure oxygen at 9m (1.9) - albeit in a hard hat but i've never heard of a cns hit from that or recreational nitrox - people switching to wrong gas at depth or ccr now that's a different story. I will agree that like anything slowly is best but i have agree that people should not be warned about nitrox as it's clearly commercialy viable for the recreational diving industry. After all.. padi hopped on the wagon in 1995 and since then it's been everywhere.