Paying to certify an extinguisher annually is silly. However, this thread has motivated me to check mine. All in the green. We go way beyond the minimum requirement, which I think is more important than excessive inspections. We have 6 aboard.
As a bit of thread drift, I would encourage everyone to watch Yachting Monthly's, Crash Boat Series, on Fire. Pop off a powder extinguisher down below and you better know blindly how to get out. It will go zero/zero visibility in one second. Very valuable watch.