I actually learned katakana and hiragana solely for the purpose of writing FAQs for the Zelda:Oracle series. Granted, knowing Japanese isn't as necessary for getting through an "Action-RPG," but it did help with some of the finer details of the gameplay. Later on, I managed to get through Shinyaku Seiken Densetsu (another Action-RPG) and somehow understood most of the story, mostly because of what I had learned from Zelda.
I've played through a few simpler, traditional RPGs in Japanese, too. Once you learn the kana for the basic stuff like "attack," "defend," "item," or "magic," it's not too hard to get through them.
For some reason, though, I find trying to translate katakana into English insanely fun, especially when the pronunciation doesn't seem intuitive. I still don't get why "kya" gets used for "character" or "captain," but then again, I don't have a very good grasp of Japanese pronunciation.
Anyway, I'm happy to hear that people are finding Mother 3 to be good. I loved the style and humor of Earthbound, so I do hope Mother 3 gets a release in the US eventually. I'm also hoping that it gets released with a strategy guide. The one that came with Earthbound was one of the best US guides I've ever scene (smelly cards notwithstanding).