I voted "No", because I think that once teams are defined these shouldn't be mixed up again anymore.
I vote to allow individual late comers if we have qualifiers that last for several games and especially if the fest will last for several weeks. Keep in mind that for a long fest, we may lose players as time goes and some will really want to jump in at after the "last" minute. At least for a couple of games, I vote to allow late registrants in and to reschedule around them.
After that, it depends on several things. The easiest way is not to allow. There is at least one way to handle late comers but it adds complexity.
Another option is to allow late players in but only to fill in voids left by those that have left [this would be like allowing substitutes].
I haven't thought too much about teams.
**** UPDATE ****
I voted "yes" but would like to change the vote to "no" under the following conditions / interpretation of "no": http://www.nuxified.org/topic/should_we_accept_late_registrations_for_the_game_fest#comment-10555