Almost every outlet in the hotels, houses, cafes, public buildings, and airports I have been in the US have 3 outlets and not two that I have stayed or used. Generally I find that older houses and buildings have the two prong outlets.
When I travel for business or fun I travel with a notebook computer with a large 20+ inch flat screen while in the hotel or the place I'm staying. I carry a cell phone and tablet that will use while traveling (in flight or stopped for rest break on long road trips). My external hard drive is USB powered.
When I have my camera gear with my I carry two 2 battery chargers that are USB powered. I also carry a large Anker Powerbank and small power bank. That's about all the stuff I need to keep charged.
For my needs it will work and cut down on weight and extra tech gear that I currently need to keep my gear charged. As of right now I only need 2 USB ports. My phone, tablet, camera batteries can charge in one of the 6 USB outlets in my car if needed. If I'm flying I would need a couple of extra USB ports to charge my stuff. But since I won't carry my flat screen with me, I could add another wall charger.
If I were to travel overseas, I wouldn't want this device, but since I'm not familiar with I would need overseas. But I do know I wouldn't a large travel tech kit. When my wife went to Bora Bora for 7 days she carried 2 adapters, 2 power banks, a couple of different cables, and a small camera and the charger. She was perfectly fine. The hotel even offered her extension cords and adapters if needed.
As to a extension cord with a 3 outlet end. Many of those cords are cheaply made, come with a 2 prong outlet, and have no surge protection. Plus my laptop is a 3 prong so I would have to carry an adapter or purchase a different cord. I prefer to plug into a protected outlet if I can.
I will admit for other users this product may not be ideal. But for some it will work.