This method is used in Mercedes dealerships, and is very popular among the Mercedes DIYers. The dealer uses it because they don't have to schlep cars to a special bay for an oil change. They simply bring the extractor to the vehicle and perform the oil change, a tremendous help for the work flow at a dealership. The DIYers like it because it makes oil changes less messy -- everything can be done from the topside of the engine.
Depending on the model, some people ran experiments where they sucked the oil out, then opened the drain plug to see how much they missed. On a lot of models either very little or no additional oil came out of the pan, proving the effectiveness of extraction devices.
I personally prefer to use the drain plug and let gravity drain the oil. But if using extraction methods will get a DIY owner to change the oil more often than he would otherwise, then this device is doing a good service and has its place in the market.