I played Ozai, the Phoenix King with 2 untapped lands remaining, and had no other cards in hand and no outlet to spend mana on.
I intended on declaring Ozai an attacker and tapping the 2 lands before blocker step to fulfill the Ozai 6-floating-mana clause. Opponent had no way of blocking flyers and was at 3 life.
The game saw i had no way to spend the mana so it went straight to blocker step with a non-flying Ozai and didnt let me tap my 2 lands.
The following turn the opponent plays The Rise of Sozin (destroy all creatures). Again I had no way to respond besides tapping lands, so it resolves and destroys Ozai.

At the time I didnt think of turning on full control because I was confused why I couldn't tap lands to turn on the Ozai ability.
Any other time I have no response is understandable why to skip priority, but having an ability on the field that cares about unspent mana should allow you to tap lands in response.