One reason would be diversification of your 'portfolio'. If something happens that grounds a fleet or causes some other significant fleet impairment, the impact is lessened when you don't run a single fleet type. Your point is valid and a good one but someone has to weigh the obvious efficiencies of having fewer fleet types with the risks of having a larger portion of your operation effected when something goes wrong with that single large fleet. In the business of moving expensive time sensitive freight, I'd be leaning towards a mixed fleet.