I think that Krokotopia should be a free to play area because the balance school students have such limited access to good spells! All of the REALLY good spells are in Krokotopia. I think if maybe, once they got to say, level 20, Merle Ambrose should send the free players there on a few easy quests mostly so balance students can get better spells!
Lol if KI did that then shouldn't everyone get to Krokotopia for free? Seriously by level 20 you know if you will continue and pay one way or the other-crowns or subscription. Granted it is technically possible for other schools to level up past 20 for free, but they require quests to other worlds to obtain their spells.
The idea of Wizard City (except Colossus Boulevard) being free is to give people a chance to see if they like the game. Mr. Wethersfield, the Balance School "teacher" in Ravenwood, has the Balance spells up to level 22. Locust Swarm and Sandstorm should be more than enough to get anybody through the free part of the game.