I chose the wording 10x more "effective", because it's not necessarily just 10x "faster", although that is what it usually boils down to in the end.
A poor developer will write code that is buggy, doesn't scale well, with a poor interface and poor extendibility. His code will be more difficult for others to use and work with. It will in the long run be a maintenance nightmare and make the project as a whole worse and thereby affect the work of many other people and require that they also invest more time in their work. (Other developers, quality, IT support, contractors that plug into the system, etc.)
In the end and for all the people involved, a project with 10 average developers might take 10 times longer and cost 10 times as much as if one really good guy just sat down and got the job done.
I've seen this happen. I've seen a one-man team create a framework that is fantastic to work with, has a simple, intuitive API and just plain works, whereas a framework developed by a team of about 20 people that accomplishes essentially the same thing is terrible to work with and buggy.
Most managers don't realize the huge productivity and efficiency gap between skilled and unskilled developers. They think firing one expensive developer in the USA/Europe and outsourcing the job to 10 developers in India* is a net win. Most of the time, it is not.
Many unskilled developers should be considered more of a lability than an asset. You wouldn't want 10 unskilled developers working on the control systems of a 747 either, would you?
If I had my own software company, I would have absolutely no doubts about who to hire. I would hire the best guy(s) I could find for a salary of 100 to 200k instead of getting a couple of decent guys for the same money.
*I'm not saying all developers from India are bad. I'm sure there are also very good Indian developers. But from my personal experience most of them are not very good, in addition to the cultural/communication problems (they will almost never admit mistakes or that they didn't understand something), I found it very hard working with them.