Outside of the BW forums there are a lot of people who haven't played a DA game before but are becoming interested in the marketing. At least some of those folks are PC players that like cRPGs. A video showcasing extended tactical cam gameplay with PC UI might convince them this is a game worth trying out, rather than thinking it's an action game because so far the game's been primarily shown in 3rd person with some tactical here and there and a controller input.
I see. Honestly, I think the Venn diagram of PC cRPG players and gamers that had never played a Dragon Age game would show a pretty small overlap, but I take your point. There are probably some.
That said, if the argument about which control method is shown in the pre-release marketing is about attracting fence sitting players I would wager there is a larger untapped pool amongst console players than there is amongst PC players, and certainly far more than PC Players who are favour cRPG games. From the perspective of a numbers game, showing play with a Controller is the right thing to do.
Even so, I was more asking about BSN posters in this thread (and elsewhere) becoming increasingly agitated and in some cases almost offended, that none had yet been shown. Other than satisfying curiosity about the UI they will be using come release, what else would this show them that the existing gameplay does not? It's obvious to me how the camera movement would be more dramatic with a KB+M and the tactical map would react differently to a mouse click than to a thumbstick moving the curser around.