actually, this was brought up in a topic I made quite a while back. I don’t know why you say the system punishes switching jobs (the only thing you lose is time, and you can switch back at any time at the same level/xp level you left at).
one of the comments talked about stat bonuses gained from doing jobs, and when you switched jobs, the bonus was reduced, so like a +2 in mind when being a carpenter, but that changing to a +1 if you switch to, say, a footsoldier. the reason you would do that is to, for example, train as a blacksmith, mason, and clothier, gaining +1 mind from each, then switching to carpenter, getting +2 from that and +3 from the others, leading to a carpenter having up to 11 mind, leading them to make a lot more fine goods. or training as several soldier classes to get a bunch of bonuses in body, and making stronger bows (or weapons/armour in a blacksmith’s case), or making stuff in less time (although maybe that can be from faith?)
the system is set up for you to stick a hearthling with a job for a long time in order to really reap the benefits of the job, but there’s nothing punishing you from switching to a new job, other than no longer being the uber job it was before. if anything, if you can max out 2 jobs, you can effectively have a single hearthling do 2 different jobs by himself, by just switching back and forth when he needs to. frees up a citizen spot when you don’t need a mason/carpenter at the moment. by allotting growth along with their skills from leveling in jobs, you can make some really good hearthlings, even if at first they were 2/3/2 hearthlings or, heaven forbid, 1/1/1 hearthlings.