No matter how dry the concrete appears, moisture is moving through it. Trapping that moisture--and under certain conditions condensation--under a wood subfloor will cause mildew and rot problems. That's why if you're finishing the floor in wood it is sensible to use a floating subfloor system such as DriCore or DeltaFL or sheets of extruded polystyrene.

The same applies to carpet over wood subfloor.

I have had success in direct gluing low pile carpet over new concrete floors (4" concrete over 5" crushed stone) in new homes. I don't use underpads, which will trap water vapour.

Sorry, but I wasn't clear. The Harmony product boasts that it has a moisture management system consisting of two sheets of polyethylene that surround a layer of flexible polystyrene granules that increase comfort underfoot, reduce ambient noise, and allow air circulation between the floor and sub-floor. This is the same property of Platon and Dricore that prevents moisture from reaching the sub floor, let alone the carpet.

I have a dehumidifer in the basement, so that along with the Harmony/Platon should, I hope, reduce or eliminate the issue of moisture turning into mold/mildew. I'm just wondering if there's a compelling reason not to use the Harmony product.

To be honest it may be no more economical than Platon. I haven't priced it out yet. I just came across it in my research and wondered...

Here is a link that might be useful: Harmony Floating Floor Underlay

Harmony says the bottom layer of plastic is designed so the water vapour can enter it and then it is supposed to escape to the sides, rather than through the top layer of plastic, which is less permeable.

If it works as stated, it's a lot cheaper than other methods. (A 100 sq. ft. roll is as low as $29.99 on line.) I would like to have seen a compression test that was run longer than 7 days.

By comparison, Delta FL and XPS need wood ply on top; Dricore and Subflor type products are more expensive per sq. ft.