> professionally printed books for example use slightly gray letters on creme paper."

Those people are fetishizing the limitations of offset printing. You simply can't produce sharp blacks comparable to an industrial laser printer with offset printing.

> I don't think you could get "slightly gray" with a laser printer

You absolutely can. But pure black on Natural Shade (off-white or cream) paper looks much better.

Most POD setups use inkjet printers for cost reasons which results in poor print quality.