Healthbars aren't the most accurate way of portraying damage. Too bad the game doesn't have "localized" damage. Ea rather then having one healthcounter every part would have it's own healthcounter. So if you hit a bodypart (say the left arm) the health of this arm drops but the rest remains "undamaged". If certain crucial bodyparts are hit (drop below X health) you die, just as if too many bodyparts overall are hit (ea you're bleeding out all over the place). Ditto with vehicles: You're turret could be all shot up, perhaps even seize to function but the tank (hull) would be alright. In such a manner you could damage (disable) skirts, tracks, the engine and so on... if only. IF this were the case you could show an icon (isometric or orthographic views) that show the (estimated) damage for each section. Ea you can see that you're left arm is badly damaged, your left track has taken a beating and so on...
But since we don't have this (tracks can only be disabled if they are set to reduce the health from 100 to 10%, at that percentage a vehicle becomes disabled thus it looks like localized damage) the silly, arcady 'one overall healthcounter' means you can only have one overall healthbar (hud icon) aswell. For infantry, well, most of the time they are killed outright so the healthbar was easily removed there. Tanks can take multiple beatings depending on where they are hit so the healthbar would still be needed there.
So unless the engine can magically be made to supported more advanced damage systems (ea taking angles into account, seperate sections and all that to determine if a shell penetrated into the the main hull and killing all the crew and blablabla) the bar serves it's purpose. The suggestion to only make it appear when you look at it with a wrench (or actual attempt to repair) it sounds fine though to me. But it will always be a trade off. Some damage one could never notice from inside the tank (ea a light was knocked off, the exaust pipe damaged, penetrated skirts and so on) but others would be notivable, be it by the nakd eye or just by the handling of the tank.