The concept of Avraham as father of our nation is brought throughout on many different levels. Avraham was also an example for equality and justice in his ways. This is an accepted social norm in today's world, and has been for generations, and is found in our Parshah. This is despite the fact that they didn't even take part in the battles, and merely watched over the supplies, as Rashi explains there. Equal portions shows equal consideration to all members of the group, whether it be tribe, family or society as a whole.
Every member has the same rights.
No one is preferred. The soldiers serving in the home guard are as much a part of the victorious army as the infantry in the front line. For without soldiers working behind the lines, making sure the correct supplies reach the correct destinations, there is no front line. Avraham Avinu showed us the way, and David kept it going. The others who had gone with him, they would recieve their due.