I am the founder of Codenvy, a type of Web IDE that is more focused on enterprise apps. We have found that the value of cloud workspaces has been around instant access and centralized control (for groups of workspaces). So, it is possible to offer on-demand, temporary workspaces without login (some vendors provide that including Codenvy). It tends to be a vendor choice as to whether requiring authentication before gaining project access is important. Most vendors do require authentication as it allows for saving important properties, user configurations, and some security settings.

To see some of the use cases around why cloud IDEs are used over desktop IDEs, we have listed them on our site. https://codenvy.com/why/cloud-ide/. The early uses have been around scenarios where configuration / setup of desktop are burdensome: pair programming, hackathons, training events, or partner integrations. We have some demos with Google, Intuit, and WSO2 that show what IDE on demand might be like, but those are special case scenarios.

As for the concept of downloading the IDE itself. Some cloud vendors do offer offline mode, and packaged chrome apps which do give that very behavior. It's first inning sort of stuff, but it will be possible in the future to allow offline sync of the development experience. Some cloud providers offer both build / debugger runtimes, so downloading offline, means also having a way to synchronize the project's files, and the underlying infrastructure necessary to execute the code. In a pure Web app sense, where it's HTML / CSS / JavaScript, this is fairly easy. But if you need to have synchronized access to dependencies, libraries, or runtimes, then the challenges are more pronounced.

Thanks for the cool questions.