4p tag battles in DoA are heaps of fun, and I think the key is that you are still very involved with the match, even when tagged out. You're watching for weaknesses, tipping your partner off, hoping they don't get beaten, hoping they can get away for a second to tap you in; you're both really in it together.
I'm pretty sure your life bar doesn't slowly recover when you're tapped out in this game, however, so there would be less motivation to tap your partner in. Man, a tag-team hack for this game would rule...