Well, a lcd backlight has a lifetime of about 20000h -> a bit over 2 years,
so yes, it's not such a great idea to keep it on when not necessary.
On the other hand frequent on/off cycles might not be so healthy for it either,
so I would say keep it on if you're just leaving for a while...
Well, if the display is native 16bit, then it can display 65536 colors. If you try to display a picture with 32bit/16.7 milj. colors then it will look strange. Something like the pictures.
Edit: Ok, I guess the external screen is also in 16bit mode on the first picture, and the original file...
I don't think you will damage the actual lcd panel if you are gentle, but maybe the anti-reflective coating. Anyway, it could go either way, you never know. But the fact that rubbing it has an effect is a good sign.
I had a Sony G400 crt and could not stand the way the trinitron tube displayed text (very unlike the shadow mask one I had before), and the horizontal damping wires were kind of annoying too. For games/graphics it was superb though.