if arc is like just like the wii mote then it is tried and true and will work just fine. I don't think arc will suck at all however it will need some decent game support to justify a purchase. same goes for natal. the only difference though is that natal is brand new and untested.
and they will market arc eventually. I'm sure they're just waiting to show some killer apps before they advertise. any advertising without something decent to show for is just a waste of time.
the only thing with arc and even natal too is that they're coming out 4 years after the wii has established itself as the motion control console. and the wii has established itself as the family console/the i have 3 other friends to play with at my house console...ps3 and 360 are mainly single player/online consoles and I do not believe that the majority of the crowd that already owns these consoles is going to be amazed by arc and natal. nor do i think the casual crowd will bother to buy arc because they already own wii and natal has a completely new control scheme that can either be hit or miss.
also people bash the wii for its gimmicky games. and they say that people only play it for 2 months and then stop. so how are arc and natal going to be any different other than graphics?