In a recent conversation in bugzilla someone asked if it was common for people using CDT to also have the JDT installed. The short answer is "yes" but that's not always because they are developing in Java as well as C++.
When people start with our Carbide.c++ they run a branded installer like you get with most Windows apps and you get an entry in the Start menu etc. We install a JRE and a bunch of Eclipse features but not the JDT. People focused on C++ development didn't want to see "all the Java stuff" so we left it out.
But people who are also doing Java development (sometimes developing plug-ins for Carbide) or who already have Eclipse configured with their favorite plug-ins can just take our Carbide features and merge them in. Corporate IT people often like to have a standard version of Eclipse that usually includes the JDT since it is seen as part of the base platform. Also some plug-ins out there assume the JDT is present and will not work correctly if it isn't. So we've got to assume there is a good chance the JDT will show up at some point.
I recall one of the sessions at EclipseCon this year where the presenter was showing Eclipse with all of the Europa plug-ins (and maybe some others) installed. The UI was an impressive jumble of widgets and completely confusing. Someone said something about capabilities being useful but added that "no one uses them."
I was surprised as we use capabilities extensively to keep the user experience focused on C++ development when people are using Carbide, mostly by hiding CDT UI that isn't used when developing apps for Symbian OS. At some point I'll have to see if we could start including the JDT but keep the UI subdued unless you start working with Java projects.