TD&A desert dragons - removing dragons with knights

I have a question about the scenario desert dragons, the scenario where dragons from the desert will conquer Catan.

The main strategy for all players is to go as fast as possible to the islands around Catan because Catan itself will run out of production due to the dragons. But there is a rule that you can remove dragons from Catan by buying and playing a knight card. We have played this scenario a couple of times and this rule is rarely been implemented - the game was almost every time over because of a player had enough victory points because he had built enough cities and settlements on the islands. That was our strategy, to build as many as settlements and cities there. The only case in which a player would play a knight card it was on the end of the game to win the game to unlock again the victory points of a settlement/city built on Catan.

I understand it, too, because you have to buy a knight for 3 resources, and then a dragon is removed, but the next time that number is rolled, the dragon lands on that tile again, so it costs you 2 resources net.

What do you think of this rule? Are there still hidden strategies?