A sandbox is basically just a cloud node with some limitations. Indeed you won't have a full DTAP street, but you can of course still deploy locally for development. It's not really intended for full-blown enterprise projects.
One application has one sandbox. If you're working on an application with multiple developers and they all want to deploy to the sandbox, this will be a bit of a constraint.
Updated: Yes you can publish web services but the application will still go to sleep so there's not much point.
You can find an explanation about what a sandbox is on our FAQ page here: http://www.mendix.com/editions/faqs/