BSP has some advantages.
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People not familiar with modelers or without skills will find BSP tools easy to use and they will be able to start making complex level design.
For example many Minecraft users are able to make incredible levels without knowing anything about a modeler tool. BSP is somewhat like Minecraft, very friendly to use and start making levels. -
They can workd directly in the editor without needing models import each time a modification is needed.
For example you have a game level with some street walls and you need to make walls with different height and some different holes, you just copy a wall and start editing it without needing to start a modeler and multiple import process.
If you look at Minecraft like games and all great levels total beginners are able to make, BSP is as easy to use and allows lot more than minecraft.