Let me just ask something...
What game are we playing here?
So looking at a red bar that is half filled its ok. Knowing he is about half HPs is ok? Though it seems that in your perspective knowing to choose between a moderate cure potion or a critical cure potion, doesn't play a part from your perspective. Well its just a difference between a 300GP potion of a 1500+ one, which is a big deal, in a game where economy plays a big factor, and players need to tactically use the available resources they have.
Now how is it that folks interpret HPs? Some tend to RP - "Ey, you seem to be wounded do you want some help?" other do it differently, but it seems to me there is this sort of common sense that HPs reflect the health condition of a PC.
Being -1HP can simply be a passed out PC but clearly breathing, while a -6 HP may just just stopped breathing, his chest doesn't move and he is in the limbo.
The game provides this option for a reason, and this sort of "Ghost" this thing about metagaming smells like a dead cat already. You are complaining some players took a tactical approach to an encounter for looking at their buddy and knowing he was not that close to death that he could wait a bit while they killed the critters before risking their own life to save him? Seems like a perfectly acceptable choice, in a game that is all about tactical deployment when it comes to combat, actually one that happens quite often, because running blindly to heal someone might just get 2 toons killed instead of one. Meta is using information that your PC could not know since it is not provided IG, but HPs have a direct connection with how the PC looks more healthy or almost dead, so its not even meta, that information is there, its just provided through numbers because the game works based in numbers. When I hit a critter i see I did 20 dmg, not that I hit a great shot...
I seriously fail to understand the big fuss around this. As was stated before knowing the HPs is just common in any pnp/dnd game.