Teams are great if they are constructed properly.
For example... 5 "full stack" developers team up.. this can be rather pointless. Sure you all can take on more work and perhaps get an extra pair of hands when you are struggling with something. However, it's all the same work each individual would be doing without a team. The only benefit here is perhaps volume and some work support. But everyone still needs to hunt down clients, deal with them, etc. There's no back and forth between team members beyond occasional project support because each freelancer will be working on their own client's projects.
A team constructed of developers, each with a unique expertise, can be a bit better - but still not quite "fabulous". If you have a "php guy", a "javascript guy", a "Ruby guy" etc that can help a bit. But in my experiences developers don't really focus heavily on one particular language. They all try to know everything as much as they can. You're kind of hard pressed to find a "php guy" that doesn't also understand javascript, etc.
A good freelance "team" involves construction the team with freelancers that have different skill sets. If you expand the areas of expertise, you expand the possible client poole and in turn the possibilities for more robust and better clients.
As an example... a great team...
- A Marketing specialist
- A Project Manager
- A Graphic Designer
- A Writer
- A Web Developer
This is a stunning team to build that can keep you fed for years and years.
The point is when the Marketing Specialist has a client in need of design services, they contact the designer... when the Project Manager needs webdev they contact the designer and developer. When the developer needs a new page layout, they contact the designer, when the design has a client wanting to do a mailing they contact the Marketing Specialist, etc.
They are all symbiotic relationships. Each doing their part to support the others and in this way a "team" can be very, very, beneficial.
The goal is to think of a project from inception to completion and try and formulate a team of freelancers who each handle a unique aspect of that project workflow. There's a gold.
Related: How can experienced contractor survive outside Elance/oDesk/Freelancer?