It's a tradeoff, fuel vs engine maint cost. To me (an old engine guy) it makes little sense to thrash those donks when it might save you a minute or two reaching TOC.
But a disciplined airline should have powerplant and ops engineers meeting with management, reviewing warranty/guarantee restrictions, sliding-scale maint. costs, current fuel prices, TOGWs, and deciding what's best for each route for the airline.
