CLI is a way of life for a POSIX developer. When I'm doing development I always have terminals open. File management, source control, compilation, some kinds of refactoring, etc. are all via CLI commands. This is especially true for modern web frameworks (eg. using the "rails" command for project management and building). Cloud9 mostly focuses on web development so it's not surprising that terminal support was in high demand. It also lets you SSH into your hosts which is obviously not that unusual. Linux servers are usually headless and don't have Xlib available.