Well... the thing about helmet view is, it never really looks anything other than cheesy in games like this. Keep in mind that unless you do have a video headset, you're ALWAYS going to be looking at your helmet as a pasted-on shape on a flat screen. If they do it so that it completely obstructs portions of your vision then that won't make much sense - most helmets have their bits in your peripheral vision, where they aren't going to obstruct it most any of the time. But then, if they do what a lot of developers do and make it transparent, it'll just look... well, stupid and cheap. Either way, it doesn't really work to improve immersion - at best, it just needlessly obstructs your view of the game.
EDIT: That said, I'd be surprised if you can't see something when you look down. This was one of the features I saw people asking for the most leading up to Oblivion, and there were quite a few requests for it with Fallout 3 as well. It's something people have been hounding Bethesda to do for a long time, and they have no real reason not to.
You make valid points, especially when concerning the video headset, however, the main pull of those are complete isolation, which all in all would be improved by helmet cam.
As for you comment about helmets looking bad, IMO in both Halo and Bioshock 2 they are VERY well done.
But still, if added allow an option to disable it ala Bioshock 2.