I agree that we forgot to remove the _source folder from inside FCK editor but that doesn't interfere with any functionality or cause any load on the server (apart form a few Kbs of additional data files lying there).
The FCK editor project itself was abandoned in favor of CK Editor. The feature is left there as it is for legacy compatibility and no work is being done on that part of the plugin through the last few updates. FCK editor section will either be abandoned or upgraded to CK editor in an upcoming version.
If you have given any attention to the plugin you will have noticed that each and every feature has an ON/OFF feature. Don't like something, turn it off and don't use it. Don't mislead people..