It is not required, do a search and you'll find a gazillion threads as to why, it isn't recommended in the schedule by Subaru.Anybody here ever change that spin-on filter on the side of their 4EAT? I'm thinking about changing it when I drain and refill the ATF.
I agreeAlthough it's not supposed to be serviceable, I can't see a single good reason why not to replace it. Just make sure you use the factory filter as it is NOT the same as a regular oil filter.