The pro's of the career being chosen is that the Elders would select on that you might be good at. The other good reason is that if you where to be spending your 6 years until adulthood on trying to find jobs that you are good at and deciding to find a job, you will have one selected for you. The final good part of getting the career selected for you is that it would be easier for people to know what you like because of the job that was selected for you.
Now the con's of this career being chosen for you is that you might be doing a certain thing because your friends like it but you just want to make them happy so you do the think like making toys, you might not like it but you want others to be happy. The other con is that you might not get what you where expecting like if you where to get a job cleaning but you only liked cleaning/sweeping, dusting and all that other stuff but you get the job to clean vehicles and other harder tasks, this would make it harder for you to enjoy life there. The third and final reason is that the Elders might get your job wrong, you spend most of your time working on building things (lets say) but outside you are playing sports, this might effect your career because you will be playing sports instead of building what you like.
If I where to have my career chosen at the age of 12, I would be panicking because I would have my career chosen and if I where to change it over the years, it would affect me alot because I might not like my future job. If I where to be 12 and have a career assigned to me is going to help me get a job in the future.