gun folks are by majority security concious, and a bit paranoid.
your 2nd reason is the reason folks do it. I believe the primary and the 3rd reason is because they have always "seen" this done and so they do it.
Problem is, if they want a semi evaluation of a gun, pictures are a must and unless they are very good at paint shop pro, it is hard to hide many of the last two, and folks naturally write luger # 34xx
This same issue gets brought up every now and then, so, I am sure there are other threads with different ideas thrown out
Ed