IGH can take a lot of abuse, so it isn't so clear that casual riding is where they shine the brightest. They seen to do very nicely on commuter bikes. There is a fellow around town here whose bike has an 8 speed Alfine and he has done Paris-Brest-Paris a couple times - I think he was on that bike. And people go on long tours too with IGH. Of course people do all those things even with fixed gear bikes!
I mostly like getting the full range of ratios with a single shifter. When I am fresh at the start of a ride I can keep track of front and rear and avoid crossing the chain, but at the end of a long ride I get really tired and my poor brain can't figure it out and I am always getting caught in awkward configurations. There are roads around here that switch from steep up to steep down quite quickly. Mostly I would want to use my front shifter with its wider steps but it is slow to shift. With an IGH I can flip quick and easy through a wide range of gears.