Very true but in a way I think things might be changing. As graphics have got better in this gen, I've noticed a lot more in game footage being used in adverts than was previously the case. In the past pre-rendered footage was always used as it looked infinitely better than anything the game engine could produce. Now the gap is marginal.
Also Ninty's current technique of mixing Wii game footage with people playing works really well. (Who would ever have thought that showing people enjoying themselves would sell games?
Having said that I wish I could remember what the advert was for I saw the other day. It had something like "Represents actual in game footage" at the bottom and was obviously pre-renedered but didn't look noticeably better than the game itself. Pointless and no doubt a decision made by some clueless marketing team who still think every game must have a FMV promo video.