Either it works only for PC and therefor should be not choseable for NPCs or it works for every Char the same wich would sound pretty overpowered. Or it will be less effective because the PC lowers the effect for NPCs. But even then it will probably be still very strong.
I tested this by keeping the extra talent you get at level 4 for all 4 characters (the main character and the 3 companions) as you don't get another talent at level 8 (I wonder when?)
How it works, as @tee said, is you get 70% bonus to vitality + 2 extra max and turn AP for the character you put it on. This is unaffected by how many companions you have recruited and can be your main character or any or all of the recruited ones. It does *not* affect the other characters though.
If you have a party of 4 and put it on one character only that one gets the bonus. If you put it on all 4 then all 4 get the bonus. Seems fair enough to me - it is just the description is confusing. It isn't lone wolf (even though it sounds like it) and the description "...depending on the number of active player characters" implies (at least to me) the party size not whether some are controlled by different humans or not.
I'll try again with 2 (human controlled) players to see what the reduction is.
Perhaps you are right and the reduction is to compensate for bless but if that is the case it seems odd as bless isn't so terribly useful you'd waste a memory slot on every character even if you had it. Also One Man Army affects AP and vitality while bless affects resistances/accuracy/dodging so it's not really the same area. I guess it will become clearer later on...
