Exact same reasoning I used for keeping the factory reservoir and the Roush intercooler reservoir. I can tell at a glance that the intercooler pump is working and the engine coolant is full. I also respect freedom of choice.
I know, I been begging auto manufacturers to put sight glasses in the diff, on the transmission, on the ac system, on the power steering etc etc but they just won't comply! I need to know the diff fluid level before I drive!