But, this is a tangible and physical differentiation between an older model and an updated one.
They were paid for in RL because they delayed the need for an entire fleet upgrade. The companies that offered the 'fixes' saw a gap in the market, and exploited them. Major international carriers decided they made economic sense...
I''m not going to argue for tiny engine improvements over an a/c's lifespan.
However, this option gave airlines and important differential in deciding whether to fleet type replace, or fleet type upgrade. Considering the economics of the former. I see this a a must addition?