There's the rub. If you put too much information in the HUD, it becomes a distraction and takes more time for the human eye to find exactly the information it is looking for. This is why multiple colors combined with a minimalist design makes BMW's implementation so effective. It gives you everything you need to navigate the streets more safely, and nothing you don't. Time to arrival, distance to arrival, and other information is good to have, but does not enhance driving safety.
As for speed display, if HUD is on, I never glance at the speedometer! I'll glance at the tach (I've got a manual transmission), but the speedometer becomes redundant. That is why I keep HUD turned off when I'm not using the Nav -- you could say I'm old fashioned and like to see the analog speedometer, just as I prefer my minimalist-design analog Swiss watch to a digital techno-overkill. There was a time when it seemed every other car manufacturer was converting to all-digital dashboards, but that trend expired quickly.
In any event, having Nav and cruise-control information holographically projected does, in my opinion, enhance safety. Voice command on iDrive further enhances safety. For example, I might be listening to the radio and I'll say:
and then go back to the roadmap display by saying:
"Map, direction of travel"
"Map, facing north".
If you use HUD and voice command (and voice command works best when you speak clearly and AT A NORMAL CONVERSATIONAL RATE -- don't speak slowly) you can keep your eyes on the road indefinitely.