"Nullifying" is a bit melodramatic. Gain a 9th active ability and you'll still have access to it, whenever you want to trade off one of the other abilities for it.
What word would you prefer?
Abolishes, annuls, invalidates, negates, offsets, quashes, repeals, rescinds, restricts, torpedoes, undoes, or voids?
Strategy is long term planning. In game terms Strategy is which allies you choose to make, who you choose to attack, and what resources and armies you choose to build. Tactics is how you employ those resources to make those allies, and attack your enemies.
Part of building those resources is when you choose abilities for you characters. The strategy you use to choose those abilities determines what and who your characters can be effective against. For example if your strategy was to choose only fire spells for mages, then that strategy would fail if your forces encountered fire resistant creatures. A very effective strategy would be to choose many different kinds of damage and many different kinds of crowd control so your forces would always have winning tactics available to them.
The limitation of 8 usable abilities per encounter does not offer any additional strategy whatsoever. In fact it nullifies (Abolishes, annuls, invalidates, negates, offsets, quashes, repeals, rescinds, restricts, torpedoes, undoes, or voids) the strategic decisions that you made when choosing abilities for your characters.
Your assertion that you only nullify (Abolish, annul, invalidate, negate, offset, quash, repeal, rescind, restrict, torpedo, undo, or void) your strategy for the next combat and that you can choose to nullify (Abolish, annul, invalidate, negate, offset, quash, repeal, rescind, restrict, torpedo, undo, or void) your strategy in a different way for the combat after that does not negate that you are forced to nullify (Abolish, annul, invalidate, negate, offset, quash, repeal, rescind, restrict, torpedo, undo, or void) your strategic choices for every single combat (once you have more than 8 active abilities)
It's the equivalent of choosing to fight only left handed or to leave your sword holstered for the next combat and only use your shield or fight with your eyes closed. It's an arbitrary limitation whose sole purpose is to reduce your tactical choices for the upcoming combat and make you less effective.