Even if you're just changing focus rather than the direction your eyes are pointing, you're looking away. There's very little information while driving which is so essential you can't flick your gaze away for a fraction of a second (you'd better be doing that anyway to check your mirrors). If traffic is that tight, you don't need to be looking at your speed, just stay with the flow. Your fuel gauge isn't going to suddenly leap from half-full to empty (if it does, you have other problems).
That said, a blinky light on the side mirror as a blind spot warning can't hurt, and maybe an unobtrusive but visible "master caution and warning" light could light up at the bottom of the windshield if some other instrument needs attention.
That said, for a fighter (or other high-performance aircraft) pilot who has to track multiple things simultaneously (where's the enemy? which weapons are armed, do they have a lock? what's my attitude after all this dogfighting?), a HUD is invaluable -- and said pilots are carefully selected and undergo a hell of lot of flight training and then a hell of a lot of training in using the HUD (and there's also an auditory component to that).