That's uber metagamey.
"Ignore the other 4 monsters that are attacking us and concentrate on the last monster".
Doesn't matter if its the monsters doing it or the PCs doing it, unless there is some specific goal to it (for example: escort mission where the NPCs try to kill the person being escorted or some such thing).
Is it a good strategy? Does it work? Yes and yes. But its super metagamey.
Concentrating fire is a classic tactic, not just in RPGs, but in real life. Maybe not in small unit skirmishes with medieval weapons, but still. It is metagaming in the sense that the system doesn't degrade the effectiveness of a creature until it dropped (in fact, many monsters become /tougher/ when bloodied), but, that's such a basic truth of the system, that ignoring would be as strange as not expecting to fall when when you step off a cliff.
There are ways in which paying attention to rules over what the rules model is metagaming, but, conversely, the rules of a game world are in effect it's 'laws of physics,' and defying them is silly in it's own way.
Concentrating attacks is the single most obvious tactic to adopt in D&D combat. It's uninspired, plodding, and trite, but it works fairly well. It's not the only tactic, though, nor is it always the best.
It's on the level of the simple truism that the shortest distance between two points is always a straight line. It is. But if you insist on travelling through mountains in a straight line, you'll be digging a lot of tunnels.