That seems way too powerful. If you're looking at +1 MIL for every single battle, the a character could pick up 12 MIL in a year just from chasing around a retreating large opponent in the middle of the map.
Well, first, the leaders are assigned from your court randomly, so this is presuming you got lucky enough to have the same guy picked every time you formed an army - even victorious armies take enough casualties in this game that, after the third or fourth battle, they have to spend a couple of months just rebuilding. So, it would be very hard to get in one battle a month, even for a pure retinue army that regenerates in the field and without any levies that need to be disbanded.
Second, this is presuming you could find an opponent large enough to field a force that you can attack (not siege) each month - but small enough that you can match or outnumber & beat him consistently. It would have to be multiple armies that you fight against - after the second or third retreat, even the largest enemy force reaches zero morale and disbands or surrenders.
Between the two, my experience has been, that fighting more than 4 battles a year is unusual. Either I win the war by then; get all my armies locked in sieges; or lose & settle peace.
Even so, I can see your point: players can "game" the system to farm MIL increases and create super-generals. This can be fixed by putting a limit on the amount that can be gained, or making it a small (low percentage) chance to gain a point, versus automatic.