This year's prize for pointless posts
I have never applied to any Japanese Airlines, although have the prerequisites.
Good on ya
I just cannot bare the thought of 6-9 months training.
I assume you mean 'bear'. There, there.
I understand it is 6 months to obtain a JCAB licence. Is that correct.
It takes about 4 months if you're type rated to get the JCAB ATPL and then a couple more months of line training/OJT (call it what you will)
Why doesnt Japan accept ICAO ATPLs and give a JCAB equivalent.
They do. They give you a JCAB ATPL after you have passed the ATPL check, which takes them about 4 months to train you for. Do you really think you could get a JCAB licence after 4 months without already holding an ATPL? Your JCAB licence is issued on the basis that you already hold an ICAO one.
Whats so special about the JCAB licence.
It allows you to fly Japanese aircraft and you need that if you're going to fly in Japan.....which you're not.....which makes your post even more pointless
Im really surprised so many Expat pilots would work in Japan.
That's nice for you, then.
Is there any use for the JCAB licence once you leave Japan, can you trade it in for an ICAO, FAA, JAR licence easily, or is it waisted ground study time?
I assume you mean 'wasted'. Who knows; we all came here with an 'ICAO, FAA, JAR' licence, so why would we want to try and get something we already have. Best ask a Japanese pilot is he's ever converted his JCAB licence to one that is truly worthy..
Just seems pointless spending 6-9 months training just to fly an aircraft Ive already flown as PIC for several thousand hours.
No, what's pointless is you telling us how great you are (because, of course, nobody here matches your unparalleled experience 
), how you've never applied and your unwarranted opinion on something about which you know very little. Great, you're qualified but you're not going to apply because you consider the training too long. Thanks for letting us know (although we weren't really interested) and have a fantastic time in whatever differing venture you choose to embark upon.