Amiga DE is going to be integral to the AmigaOS, so on amiga machines you wont be running such programs in amiga De on top of another OS. As I understand it, amigga DE has, or will have java support as well as many other languages.
The reason why you would develop for CPE is obvious, you only have to write the software once to have it available across many hardware and operating systems which saves valueable time and resources in porting said software. It means you have a bigger potential customer base to sell that software to, which translates to bigger sales.
AmigaDE and AmigaOS at this time are seperate entities, but the roadmap described many times over has Amiga DE being integrated with the OS which means the OS SDK will eventually cover Amiga DE as well.It is also my understanding that Amiga DE is where all the AG2 technology is being developed and with its integration with OS4 will bring that technology to the OS culminating with OS5 when all hardware dependancies are eliminated.
Anyway, to answer your questions briefly: Larger available customer base without the need to port your applications, it is not restricted to Only java, but can make use of many different styles and languages,and it is capable of running large complext programs depending on what the hardware it is running on is capable of.
Anyway, I have two points for you to consider:
1. Even if DE turns out not to be all that much better than java, it at least has the potential to attract some developers to it that would otherwise have zero intrest in the Amiga platform thereby gaining software support for the Amiga by default.Even if it is only stuff like an IRC client or nifty mps3 player it adds to the platform.
2. If Amiga DE is simply not going to be capable enough for these larger type programs, or burdensome to develop these programs for and so on, then the fundamental design as the base for OS5 is flawed and doomed to failure.WHile it is possible for the guys at AMiga Inc to have made such a blunder, many of them have been at the very least extremely competant and in this development for quite a while. Surely even if they had not noticed this flaw, then the guys at Hyperion working on OS4 surely would have as they are building the other half.
While I agree that an SDK as you have described is essential, and we have been assured that this will be provided at some point, the nicest development platform in the galaxy is not going to get major software houses to develop for a platform that only has a few thousand people as userbase.At best we may get some ports, but even that is difficult to make a profit on as I am sure the Hyperion guys will tell you.
I am no programmer, so I really cant give you the nitty gritty you desire.However, the intrest it has atracted from such companies as Sharp, Nokia, Sendo, and even microsoft and Genesi says there has to be something to this DE thing. I may not like Genesi, but those guys are no-ones fools and it seems to me the long running dispute between Genesi and Amiga Inc would make it less likely that Genesi would want Amiga DEas it would end up helping Amiga Inc if Genesi did get it ported to thier platform.
Maybe you ae right, maybe I am being overly enthusiastic abut it, but on the other hand, maybe I am not.