Some airlines, ie BA and Flybe have introduced the JOC as a pre-typerating bridge course in order to bring the cadet 'up to speed' with the environment in which they will be operating. It also gives them an idea of how they will cope with the rating, and therefore has been used as a process of elimination to those judged to be below the grade. Similar thing happens at CTC as well.
IMO, this is all very nice if you don't have to fork out for one and also have a probable job at the end of it. It extemely doubtful that an unassessed, non-airline job affiliated JOC will do anything to bolster your CV because as like the MCC, its not a pass or fail course. I managed to pass a 737 TR without a JOC and I had only experienced 2 hours in a jet sim prior to the training. Therefore I'd stay away from the JOC and save your money, especially with today's employment market. If you do have a sim assessment, hire a sim for a few hours. This will give you an idea of your ability.