The problems arise from the fact that either the planning time won't be enough, or too much for impatient players. A solid tactic cannot be established between people who have never played before as teammates, and you surely can't expect a bunch of people you don't know wanting to actually help the team rather than playing last action hero.

However.. there might be servers where people who know each other get online more often.. at such cases an option like that would be beneficial (although late joiners will miss the briefing).. but in large public servers?.. I don't believe it would save the game more than it would cripple it.