I really can't see any harm in lifting the restrictions on some classes (or making them time-based if lifting them entirely isn't an option). If someone wants to make a courtier or a priest or a diplomat straight off, what's the harm? To make sure new players know about these classes before they try them out? Well, no offense, but this game teaches you pretty much nothing about the mechanics of classes other than the one you're currently playing (and even then, sometimes not a lot. How many players who've just made their first Hero know they can tell tales or recruit volunteers, given those actions only appear a very small amount of the time?).
I've been playing this game for nearly two years, and another two years the first time round, yet I know pretty much nothing about the mechanics of the diplomat classes or the options available to it, because I've never played one. I knew very little about the options available to priests until recently, until I asked here on the forums. And that's just it. This game teaches you buggar all about stuff like that. You learn from other players. And if someone really, really wants to be a priest, they'll likely ask someone about the details and find out details that way. Having these restrictions just prevents them from doing what they want to do, even after they have some information on the class they want to be.
And, to be honest, telling someone to pause a character in a realm they might really like, but is peaceful, is just not the way to go about increasing player retention.