Any additional task to accomplish in a high workload environment is adding to the risk of forgetting, omitting or incorrectly execution. With packs, this can be potentially very serious.
I am talking B737 here, not Airbus or better designed Boeing types.
As I said, fuel saving is a good practice, but we are becoming absolutely anal and overdoing it when a company cincreases workload, complexity and risk for a negligeable benefit.
Airmanship oversteps economics. Always.