Seems like U and Black are usually intended to discourage a large fleet, frequently of smaller boats, from being too aggressive on the line. Sometimes the PRO will put up with a general recall or two first, but often if an aggressive fleet is anticipated they’ll do the first sequence under U and if there’s a general on that they’ll go to Black. And I’ve seen events where the question is put to the competitors in the initial PRO briefing and the fleet requests U-Black. That either calms down or eliminates the too-aggressive starters.I understand the frustration RC has with getting a start off sometimes, but IMHO, going to the Black Flag is disrespectful of the time, money, and effort it takes for sailors to get to some of these regattas. It devalues the experience.
Mainly out of respect for the boats that would rather do one good, fair start than five general recalls.