There's a certain logic to this, but the math would be off -- now a monster that should reliably kill a soldier (base 6) when acting in a group of five (basex4) does less damage than one acting in a group of four (5 damage vs. 6 damage). I'm not saying any of this has to be realistic, per se, but internal consistency would be good.
For me the logic is that the groups organize themselves organicly into groups of relitively even power. So a bunch of weaker monsters will group together while the more hard-ass will have smaller groups.
It also means that this soldier-killin' monster doesn't actually pose much of threat to that soldier anymore. The average 1st-level Fighter is likely to have around 23 HP (assuming a 15 Con, which isn't too unreasonable). Now instead of potentially one-shotting that guy, the monster will have to damage him four times to kill him.
I don't see a one-shot kill being a "fair fight". Two to five attacks with the armor and HP to say in the fight fits my definition of what a fair fight should look like. I also take into account what the group should be able to handle not just one individual.
And if "soldier" doesn't have any correlation to "Fighter," then gauging the monster against a "soldier" doesn't really help me figure out what it can do. My monsters aren't going to fighting vaguely defined NPCs -- they're going to be fighting PCs. If I can't draw an equivalency there, then maybe it'd be more helpful to use something like character level instead of that "base" number. In fact, if I need to get an idea of how a monster stacks up against PCs of various levels, I might as well just assign the monster itself a level OH WAIT.
I'm all for giving monsters levels and using the questions to modify that base but for now I'm going with what is written. I am almost certainly going to have to "de-level "one of the monsters I created today to make it fit into an adventure for first level players. Other wise it is completely invulnerable to the party (6 Armor and 44 HP)!