SOPs for reduced-engine taxi in/out will be designed based on manufacturer's recommendations for idle times contributing to max engine health. And yes, there are important economic benefits attached...the bigger the fleet the bigger the economic benefit. Just for example, the JT8D-219 needed 2 minutes at idle prior to setting T/O power...and one minute at idle before shutdown after landing.
We even did single-engine taxi in/out on the B727 when circumstances allowed.
As Havick mentioned, there are several very practical considerations to NOT doing reduced-engine taxi. The captain will decide when/if.
Blow over a bunch of baggage carts/injure ramp workers doing reduced-engine taxi in/out and see how far you get telling the boss how much money you were trying to save the company.