Tor Iver Wilhelmsen
You can't play an ability where you have to increase knowledge or lower morale as part of the cost. It's totally ok as part of the benefit.
This is common to many other games as well (including Scythe, Terraforming Mars, ...): a cost is mandatory, a benefit is optional. E.g. in Scythe you can take a bottom-row action just for the coins or enlist bonus even if you cannot do the "real" action because you e.g. have built all your buildings. And in Terraforming Mars you can play a card that increases oxygen even if oxygen is at max - you just ignore that part of the effect.