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Two communities developing two almost exactly identical pieces of software instead of just one; makes both communities weaker and makes progress slower.
And how is it different from proprietary?
There's more than one proprietary DB implementation, in games more than one FPS, MMORPG, many raster graphics programs, C++ compilers, operating systems, Media players, etc.
Add to that there is a lot of dead wood lying around everywhere. the code in OpenOffice will be picked upon for LibreOffice. LibreOffice will be the fork of choice until some other fork comes along and Libre becomes dead wood.