As someone who has been working as a contractor for the past two years, the big thing I see missing from that list is 'the ability to say no'. That is my primary motivator for staying independent, I get to pick the problems I work on. That is the deciding factor when switching clients or picking up a new task with an existing client, which one of them has the most interesting problem.
The big negative that the article doesn't talk about is all the time you have to spend maintaing a social and professional network. The interesting problems aren't going to come knocking on your door.
Also, the comments about getting paid for 'all your time' are BS. Submitting invoices for 60 hours a week is a great way to find yourself out of a contract real quick. That extra billing is reserved for things like crunch in the last few weeks of the project or when someone explicitly asks you to take on extra work. Invoices over 40 hours maybe cool with the manager, but they send up red flags in accounting and suddenly everyone is wondering why they are paying you for 60 hours a week when they could hire 2 or 3 full time employees.