tdracer,
On the larger 4 engine aircraft (747 and A380), using the outboard engines above idle during taxi can result in nasty consequences to those behind as it throws out dirt, gravel, etc. from the edges of the taxiway.
Not only that, the outboard engines that overhang taxiways can also suck up both small and larger particles depending on the degree of throttle advances above idle. Both can damage engine components.
The first DC-8 aircraft which were re-engined with CFM56 engines began to show HPT blade distress (burned blade tips) very early in their lives on outboard engines. It was found that fine sand was clogging the turbine blade cooling passages at the blade tips. The fine sand was traced to taxiways at KPHX. Even although there were air-particle filters in the engines to prevent contamination, this particular unique sand consistency made it through and ultimately caused the tip burnout problem, Murphy's Law at work. The solution was to enlarge the blade tip dust holes which corrected the problem.