I'll never remind someone if the card says "May" or if they forgot to take a action to give it to them later. At the last tournament I went to on Sunday, at the very end of the game i told my opponant that he didn't use whisperers ability once, I realized it half way through but i felt it was his responceblity to take the free focus. He didn't mind at all and it seemed he would have done the same thing. He was accually happy that I told him so he could remember in the later rounds.
You're not wrong. Sometimes though it depends on how much you want to push your current build. Would you rather test it against a fully effective Whisperers or a gimped one? Then again some people test FOR tourneys specifically not the continual evolution of a build or strategy.