I wanted to add here (due to comments on the other thread) that there are two principal reasons that unlimited characters can potentially be very good for an OP campaign.
First, there are rules-monkeys out there who just like tinkering with character builds, and only really want to play a character a few times, and have no real desire to track xp. They can create a vast stable of unlimited characters and try stuff out to their hearts' content.
Second, there are players who don't get to play very often, and so are liable to get left behind in the "xp race" that naturally comes to afflict any OP campaign. The players who start first will generally always be ahead of everyone else, because they've played everything first. Eliminating scenario retirement times takes a step toward remedying that, but Unlimited Characters take the big step, allowing the occasional players to stay level-consistent with the players around them. In this case, maybe somebody only wants to play Zark the Magnificent (because they like the concept, perhaps), but only gets to play a few times a year when they hit a quarterly gameday blowout or a local con. As an unlimited character, Zark the Magnificent will be competitive and valuable at any table, and will never have to take a back seat simply because the other players at the table have played more, and their characters are higher level.