I think some of the question is that 40k and the plethora of other games GW have released has set a precedence on how to interpret true line of sight to keep it simple and easy. A lot of us have simply gone "that's how GW are expecting us to interpret it", whereas there are also a portion of people who have said that the rules state a model in a unit must be visible. I don't think it's worth a massive debate/argument as I think both sides are equally valid.
From a thematic point of view, I believe that the majority of the model must be visible - banners, cloaks, weapons aren't valid targets (ask anybody who has ever done reenactment or LARP).
However, most importantly it's the choice of how the players wish to interpret it - it doesn't need a house rule, just a bit of communication. The first time in a game it occurs you just need to speak to your opponent and ask "are you happy that they can see that unit?"