If young people wish to spend a few years teaching in China (so long as they're qualified) it is perfectly ok to do so. It's only when they are unqualified that it may present a problem.

Take me for example. I worked in China for two years. In that time I saved a little over £10000. I was earning 10000rmb a month and it was easy to save what I did and still live very comfortably. I could have saved much more had I been more thrifty! After I decided to leave China I walked straight in to a job in the south of Italy, because I am a qualified and experienced teacher. I will remain teaching for the rest if my working life and so getting a job is no problem to me, I can pick and choose.

Now, if I were not qualified to teach (but perhaps qualified in marketing as per your example) and I chose to go to Asia and teach for a couple of years in order to experience life in China and chase Chinese women, not because I want to teach, then yes that is a silly move. While all your marketing peers are out getting experience in marketing, you're doing a half-assed job teaching Chinese kids conversational English and counting the minutes until you can be back out pissing any hope of savings up the wall on cheap beer and hookers. You then realise how hard it will be to get a job back home as you have no experience in your chosen field and most marketing companies are not impressed with "16 lessons a week teaching the same lesson repeatedly" as valid experience, nor does it show great work ethic!

So unqualified teachers will have a lot of trouble. But then unqualified teachers using the position of teacher for a visa only have themselves to blame. Qualified teachers will have no trouble leaving China and securing work elsewhere.