In theory it's a good idea, but if this ever happened then the maximum members of Gang A would all stay in the flag room defending whilst other members of gang A who are OFF TAG go into the base and attack the attacking members without any gang tag on. Big base battles don't happen that frequently so I definitely see the bigger gangs doing this.
Yes it would still work slightly but because of what I said above, the drasticness of this idea does not warrant the time being spent implementing it and having to deal with backlash with the small amount it would even help because of there being many ways to get around it.