Of course they are not going to tell you to look out for that stuff because they don't want people to think they should worry about flying. But they are relying on that if you do see something self preservation will have you mention it to the flight attendants. No, not everyone will be looking outside, but then you do have people like me who insist on window seats and if I'm awake, I'm gazing to see what's going on outside (I like having a view of the wing even,I tend to pick seats that give me a good view of the wing and of below). But then, I get distracted by my gizmos and aren't looking outside anymore, I don't notice. More people like that, less chance *some one* will notice (it doesn't have to be you).
Anyways, I'm pretty sure it's mostly to keep us alert on the times the plane is most likely to crash (landing and taking off). I tend to ignore it anyways if I can get away with it when it comes to playing my music (The airline attendants don't notice) but I also do actually make a point to try to stay somewhat aware during those times (if I hear an announcement going on I do try to lower the volume some to hear if it is important and I do try to keep some attention if they are announcing something).
And yeah, the cellphone doesn't do much to disturb the avionics. At most it might interfere with a navigation instrument that might be a problem if you are trusting it and not realizing it is getting interfered with. That and it makes your headsets (My roommate flies and flies me with him) get this annoying buzz sound if the phone is trying to send out signal.
And once again, to the people who think that phones will explode if you keep them on, why do you think they then allow laptops and ipads and ebook readers to be used on the plane if you think having a device on during flight causes its battery to explode? In fact, read up on the guidelines on batteries you bring with you. They want them installed in a device or at least the battery ports to be covered to prevent a short. Also, spare batteries not put in your checked in bag (if something happens they will not realize it if it is in the checked in bag). They're actually not allowed in checked in bags. But it has nothing to do with whether a device is using them, in fact they prefer them to be installed in a device and the batteries they are worried about are the spare batteries that are not installed (I think you can check in devices that do have batteries installed but most things that would have batteries installed are recommended taking with you for "valuable things you don't want lost/stolen").
