[Tyhm]

1) Hackers. They can't memoryedit from the editor directly into the online exe by swapping spaces for the runtime of one with the runtime of the other, or however they do it now.
2) Speed. The EXE will go faster without the editing tools taking up useless space (well, probably not, but it'll certainly download at least marginally faster), and the editor will go MUCH faster in loading if it doesn't have to keep a mind to any runtime things.
3) RC. You can more easily associate .graal/.nw files with graaleditor.exe than you can with graal.exe, as graal.exe tries to open one level per instance...meaning if you open 20 levels with RC, you get 20 clients. With a seperate editor, even if they're all in their own windows, they take a negligible chunk of memory.