The problem with the .500 in conference record is that, like the teams themselves, conference aren't all equal, sometimes dramatically so. So a team that finishes a game below .500 in Conference A could be superior to a team that finishes 3 games above .500 in Conference B. The arbitrariness of the standard means the team from Conf. A cannot be in the field while the team from Conf. B might be in. In fact, the team from Conf. B could make the tournament specifically because the Conf. A team was excluded. While it might be silly to include teams that didn't win at least half its conference games, it would be even sillier to exclude them on that criteria alone.