I would honestly prefer the driver to always wear the helmet. It's easier just to imagine the driver's face, and the less faces in the game the better because they ruin the realism IMO. While PD are among the best in digitally recreating cars, they are not so good at humans (for example Jeff Gordon and Vettel looked hilarious and almost creepy in GT5). Of course I can't blame them since the focus of the game is on the cars, but why force something that brings down the realism factor? A voice over would have been enough.
Also, some people would probably want to customize the driver's face and while I'm not against that it means PD have to spend even more time on this feature. Driver customization with the suits and helmets is more than enough for me. So basically, I think this is something PD should not mess with but if they have to, then do it after more important issues have been resolved.