[AnimalMuppet]

The problem with those units is that you can't start a new load in the washer before the previous load finishes in the dryer, because there's only one drum. So if you want to run several loads, it takes twice as long.

[MoosePirate]

But you could have two of them in the same area.

[khedoros]

If you've got space for 2 machines, you can increase energy efficiency by using the specialized machines.

[mikepurvis]

You waste a lot of energy heating up and drying the drum, too, compared with a conventional dryer, where you're only drying the actual clothes.