Workbench is okay. It does fine. But the code inspection is pretty much no better than the FE, it eats a lot more resources, and is generally clunkier and less extensible. An all Mathematica IDE can leverage people's talents at writing Mathematica code to make Mathematica plugins.
Now, this is maybe never going to be quite as good as the IntelliJ plugin which as actually a very nice piece of work. But I think working within the FE provides the potential to make a really good Mathematica IDE--if the FE can be wrangled in just the right, highly-undocumented way.
