Are you saying RJB that April 2 is the last chance someone has to throw their hat in the ring, or that it would just be good form?
Trying to put myself in the position one of those who didn't enter into the primaries (because of finance, or to keep your powder dry for 2016 - conceding Obama will be a 2 termer) you'd have to be a master strategist to have guessed the lack of a decisive front runner in the primary at the time everyone announced their candidacy. So I'm guessing that it wasn't the long term plan.
So, a small group of potential dark horses are working with very small teams on possibilities, triggers and strategies, about what needs to happen to make a sudden entry potentiallly successful and at what time. Beyond my brainpower but sounds like a great collection of episodes for the West Wing.