You've misunderstood me...because I didn't make it clear.
Again, a little posting in the evening without drinking enough coffee.
So...
Try this:
-Open up UEd and subtract a cube
-Add in a nice cylindrical pillar into the middle of the room. It should touch the floor.
-Rebuild
-If it is solid check the Z/P view and you'll see a lot of cuts on the floor. This will mean a higher nodecount in the game, more complex BSP structure and you'll get a bigger chance of having BSP holes.
-Turn it into Semi-Solid
-Rebuild
-The cuts will disappear.
-If you check the pillars interior in ghost mode you'll see that when the pillar is solid the surface where the pillar touches the ground is removed. If it is semi-solid then it's still there.
It's like having a couple of sheets there which have collision. This is why they can be used for ICHs without having the edges separated from the opening. See, you have the invisble collision hull which normally would remove the touching areas, leving BSP holes/HOMs but as you've used Semi-solids the touching surfaces will remain there. So the main concept is that they are added on the end of the GeoRebuild phase and the don't really affect solid geometry.
You would say Great, then why don't we use them all over the map.
-They can't be used as trim. If the outer surface lines up with the solid surface it will be removed leaving the Solid surface there. (No effect)
-No subtractions through them. (They are applied after the Subtracted brushes...)
-They don't create cuts on solid geometry but they do create on other Semi-solid geometry and on Non-Solid Geometry.
-If they are used along with Non-Solids they can interfere easily and will cause solidity-loss on walls/floors. (You'll be surprised when you fall out of the level

)
-They can't define zones.
-If different parts of the brush fall into different BSP Hulls then some sides will be missing. (They are fixed during the Optimalization phase most of the time though...)
That's it...I hope it made things clear. If not, then ask what's not clear enough.
(I hope I haven't forgotten anything...

)