I think the strongest argument for making the battles best-of-5 is that it takes a lot of preparing and planning to make one round happen, and if all that leads to a quick 0-2 loss, it might be very disappointing to that player, especially if that player is the first one to be kicked out when the team can only build 7 decks.
I would also like to add that most players would probably accept 3 losses as getting beaten fair and square while 2 losses could be seen as a fluke, regardless of what the actual math tells us.
Problem is of course that this would mean a potential 67% increase in total games, going from 3 to 5. I personally don't think that's a big deal, but maybe someone else disagrees
Everyone vote.