Namek Well, not true exactly. Jared has some good points, but the points you brought up are irrelevant.
Let's set up a situation here.
The Corporate Sector Authority (CSA) controls 15 systems. They are allied with the - Pentastar Alignment, Eriadu Authority.
The Hapes Consortium controls 13 systems and has about the same or slightly smaller military than the CSA. They are allied with no one. The Hapes would not attack the CSA, for that would be suicide. The CSA could attack the Hapes without calling its allies making it a nice fair war.
I say if you are the aggressor and have more allies than the enemy, and your allies would devastate them, you can call them in, but it would make a nicer RP if you didn't. If you were attacked and have more allies than the enemy, sure you can bring them all in, because the aggressors were the ones who initiated the conflict and absolutely destroy them.
Of course, people don't have to follow these principles, but it would make RP much nicer and cleaner without all the stacks of ships on one or both sides.