While Harry's coat is narratively an "always on" item, so it would make sense to make it work that way, I'm really agreeing with the final book on not allowing any items to be always on, it's just way to powerful. Throw in an extra slot for frequency, along with a specialization, and you're not really likely to run out of uses in a session; worst case there's always to "extra uses for 1 stress" rule. Looking at the books, Harry doesn't rely on the coat all the time. It usually protects him from surprise hits, and the rest he brings up a shield for.