"what is the proper name for e.g. Eclipse or any other "IDE" where parts can be replaced?"

I'd call those kinds of IDEs "frameworks". The best examples are Eclipse and Visual Studio, where the IDE is an open platform (and yes I'd include VS because Microsoft does provide developers information on how to link in tools in the MSDN program) that gives third party tool providers a common interface to integrate their development tools.

Of course IDEs aren't necessary. Most of my programming career, from ancient Univac 1100s up to VMS systems a few years ago, all I've ever used were command line interfaces (well, with punch cards or ASR33s using a GUI was kind of a moot point). Does an IDE save time? You bet, and that's why I don't go back to CLI's. A modern PC has cycles and disk to burn, so I have no problem making the computer do all the work, or as much as possible. My time is more valuable to me, and more expensive to the company, than the cost of a fast PC.

I became a convert when I started using the VMS software engineering tools provided by DEC (now HP) for VMS developers. They were command line, but had all the essentials: code management, language sensitive editor and performance testing. Those tools were complicated to use and took considerable time to learn, so much that I saw other programmers in the same shop not make the effort, to the overall detriment of the company when they left behind a mess of directories full of partially tested code with no history as they moved on.

Today all those same tools are as easy as clicking icons in VS or Eclipse. I use Eclipse as much as possible, typically for MSP430 and ARM projects. Erik is right, Eclipse isn't ready for 8051 yet, no debugger, but it's getting closer with the open debugger project in the most recent Eclipse release. Once there's a working debugger for SiLabs 8051s I'll switch to Eclipse and erase the marginally useful SiLabs IDE.

Erik's well known distaste for IDEs is justified by his comments, and I don't disagree with his previous statements, but I don't consider them relevant either, because they are too narrowly focused on the technical side (trees instead of the forest). There is another side to programming, especially embedded, and that's the economics of a project. An IDE is a way to reduce programmer costs, by bringing in junior level programmers who aren't comfortable with CLIs. Train them? Sure, but that takes time and money, and it's bucking the trend to rely more on tools and less on skill.

Perhaps Erik is blessed with large budgets and an unlimited pool of skilled programmers all with 10+ years experience. I don't have that luxury, so I look for tools that allow me to make better use of the resources I have, focusing on project development instead of command line syntax.
Jack Peacock