I do agree with you that a debuff wouldn't be enough to completely deter people from leaving warzones early, however I think you're missing a point...
I think what most of us want to stop is players leaving early BECAUSE they are losing or the group sucks, and immediately requeing and hopping in a different battleground. In this specific scenario, a debuff IS ample punishment and I'll explain why. A 30 min debuff that prevents them from competing in another warzone would take longer to wear off than to just finish the warzone that they left in the first place.
The people we DON'T want to punish are the casual gamers with busy lives. If you overly punish a casual player simply for being a casual player, they will quit, unsubscribe, and go play a different game that DOES cater to their lifestyle. While this may be perfectly acceptable to you, I'm pretty sure it's something Bioware would like to avoid.