Alright, a quick explination of a BSP:
Pretty much every 3D engine uses a technique commonly referred to as the painters method. This means the engine orders all the polygons in the scene from back to front. It then draws the polys, from farthest to closest, so that closer polys appear over top farther polys, creating perspective. Unfortunatly, there are a few problems with this, which I won't get into now but the main problem is speed: The engine has to figure out what to draw first, what to draw last, etc. And a lot of time is wasted drawing polys that aren't even visible from a certain position. So the BASIC point of the BSP file is that it tells the engine which polys to draw, depending on the camera angle. This is much faster, and as a nice side effect, allows for fast and easy collision detection. More complex engines like Quake/Half-Life use BSP files that are compiled like so:
Process 1: CSG: Removing un-needed polys. Picture a submarine: From the inside, you can't see the outer hull, and you can't get outside because the interior is sealed. Kinda how BSP works. Because you will never be able to see that exterior polys, they are removed. That is why BSP's must be entirly sealed, or else the program won't be able to determine what is "inside" and what is "out".
Process 2: Making a Binary Space Partitioning Tree (A BSP)
A BSP basically gives the engine data, so it doesn't have to waste time on calculations.
Process 3: Vis: Calculates what is visible from where... I think
Process 4: Rad, or Lights. This takes all the lighting data and compiles it into the BSP, giving the Engine all the lighting info, again so the engine doesn't have to calculate it in real-time.
Of course, thats not exactly how it works, but I tried to put it in a way anyone could understand.
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So, the quality of your map file is decreased, but it's for the better most of the time

(By decreased I mean removing uneeded polys and the like)
The thing is, textures and lights are not calculated byt the included BSP convertor, from what I understand. Just CSG, and Vis I think.
Alex Wanuch
aka rapscaLLion
Kousen Dev Progress >> Currently Working On Editors