Two things here:

1 What if your forced to quit to attend to some sort of emergency does this still apply to you?
2 Does this apply to someone that doesn't want to leave but has to take a $hit and gets forced out?

The current problem highlights the risk-free nature of both joining battles and being able to leave them on a whim.

Why not make it so you can only bring X amount of gear to each match, always including Militia Starter BPOs by default (they can't be lost in this example). Lorewise, are you going to bring your WHOLE armory with you to every battle?

• You quit, your stuff goes up for salvage as gear or straight isk value to your team at EoM to help reimburse them for your sorry a$$.
• You lose the battle, you keep what you kill/help kill.
• You win the battle, you keep what you kill/help kill.

This creates four incentives:
1) Buy BPOs to give CCP dosh to risk less gear.
2) Makes quitting hurt the pocketbook & isk-pocketbook
3) Makes an incentive to play a sh!tty match in order to keep your team's deserter's stuff.
4) Puts a downward pressure on metalevel and expenses of gear brought to each match in case you DO end up wanting to leave, or even stay and fight.

Further down the line, you could also set something up in PC where if Corp A launches an offensive against Corp B's armory, you could have some BIG isk value up for grabs if A can only bring their best gear while B gets to throw everything they have at defense.