Hello fellow expat in Japan.
I would say going for Finnair Platinum is totally worth it. When you go from zero to Platinum, you will basically earn 7 long haul upgrades. 1 long haul upgrade when you get gold, three more when you reach platinum, and by then you will have earned about 150 000 points, which is worth three upgrades. And this is just when you each platinum. By the time you reach platinum you will probably be close to the next period. As soon as you roll over to the new period, you will get three more long haul upgrades. So shortly after the first year you will have made more than 10 long haul flights in business.
Also, when you fly JAL there is a high chance of getting upgraded. I got upgraded to business when flying to Beijing recently, because me and my travel partner could not sit next to each other in economy. Same thing happened on the way back, when lining up to board the plane in HKG the JAL staff approached me and told me they moved us to Economy Premium.
Last time I flew AY to HEL from NRT, the JAL staff at check in desk upgraded me and my girlfriend because her seat got taken. The girl at the check in desk called her boss and was very worried that they might offend a OneWorld Emerald member. Solution: Upgrade.
I discussed the perks with a friend who just became Diamond at SAS, and I must say, Finnair Plus is so much better.