You need to name the remote, so you could do:

git remote add sourceware git://sourceware.org/git/binutils-gdb.git

I did the conversion. The reason I made them a joined repository was partly historical and partly practical.

Historically gdb and binutils have pretty much always been together. They were in a single source tree (called "devo") when they were maintained inside Cygnus. Then, later, when sourceware.org was set up, they shared a repository (called "src"). You may not have noticed this because the repository used CVS modules to let developers check out just a part of the tree.

Practically, gdb and binutils share a lot of code. They share their build infrastructure (configure and the like); they share support libraries (the libiberty and include) directories; they share the BFD library; and they share the opcodes library. To me it made more sense to keep them together, both to avoid constant merging back and forth (this is already done for some components with GCC, and it's a real pain), and also to try to minimize problems where changes to one project negatively affect the other. For example, at least in theory people doing regular development on BFD should build both gdb and binutils.

The top-level configure script used by the shared repository lets the developer disable any particular directory using --disable-DIR. For example, if you don't want to build gdb, pass --disable-gdb.