There is no proof to suggest that subliminal messaging actually works, but it is still considered taboo to employ it, as it affects free will (shops tried it with music at one point, and although their sales didn't increase significantly they still pulled the idea as immoral). On computer screens subliminal messages could well spark eplieptic fits and damage eyesight (you try focusing on a flashing image and see how soon your eyes boggle) and it would be too expensive and difficult to implement.