YANBU, it is a matter of basic courtesy to look up while the cabin crew go through the demonstration. You don't have to pay attention if you don't want to, but stop talking, put down your paper and at least pretend you are listening. If you actually smile at the cabin crew as they are doing it you might even get slightly better service from them when the drinks come round.
That's the etiquette point. Turning to the practical reasons to pay attention, I know a fair bit about this because it's relevant to my job. If you don't listen to anything else, do pay attention to where your nearest exit is because that will of course be different every time. To those who say "well, if the plane goes down we're all dead anyway", you should be aware that crashes are not common but emergency evacuations either before take-off or after a safe landing happen a lot more frequently than you might think. I have 2 on my books at the moment - one when an aircraft landed very fast and heavy after engine failure, so the passengers had to be evacuated quickly in case there was a fire, and another when the passenger cabin filled with smoke before take-off. These were safe, reputable, major international airlines.
If the aircraft has to be evacuated and you don't know what way you're going, or know to look for the lights in the floor, you are endangering yourself and other passengers. Yes, people do panic, so they'll be getting in your way, and the cabin crew won't be showing you the way either becuase they all have to get to the doors and man the top of the slides once the evacuation begins. Also important to be aware of the instruction not to inflate the lifejacket before you exit the aircraft. When you get to the slide, listen to what they tell you too - the most common injuries are broken ankles sustained on the slides.
Re the oxygen masks, sudden depressurisation is rare but it has happened, though I accept that it's fairly obvious what to do when the mask drops down. However how many people have panicked that the mask isn't working because the little bag doesn't inflate? If you listen to the briefing you'll hear them tell you that is normal.
And by the way I always keep my seatbelt fastened throughout the flight regardless of whether the fasten seatbelt signs are on because I have seen countless cases of back and neck injuries sustained by passengers slamming into the ceiling during unexpected clear-air turbulence. The other day a guy broke his ankle in such an incident and the whole flight had to be diverted to ge him urgent medical assistance. Yes, you do have to get up now and again to go to the loo, and you'll just have to risk it when you're walking about, but it's really not that much of a pain to fasten it when you sit back down.
And if you don't fasten your seatlbelt when the sign is on, and the crew endanger themselves coming over to help you, the airline can sue you.