If I select a block as a detail block, the block will split geometry if it intersects non-detail normal blocks during runtime. This causes unnecissary splitting of the surfaces for the BSP tree:

"Detail blocks must not intersect other blocks"

"Detail Blocks do not suppress the rendering of the scene behind them, they should not be used for objects that hide much of the level"

Since they arent calculated within the BSP then they aren't culling anything behind them, so why do they split the intersected surfaces? Arent they just considered as models then, just with an adjustable/tilable skin, since they dont suppress rendering?

For instance a window pane that outlines a square port in a wall block. I could make the window pane a detail block, but the only way for the block to not split the surfaces is to make it not touch anything by make it float in the air. This is noticable even at a resonable distance if the snap number didnt bring the detail block on top of the main block.

I know I could just use an MDL for the window pane, but it will take a long time by trial and error to get the dimensions to match the level block's window size. Plus I would have to make different model files for different sized ports.

Am I missing something?

Detail blocks interfere with shadow calculations. If there's a risk that a detail block is in the way of a shaded block then the detail block will be converted to a regular block.
Solutions:
a) make sure that your detail blocks don't cover up large portions of shaded blocks (in particular avoid the center of shaded faces)
b) make your shaded blocks flat instead
c) use the slider in the map compiler to force detail blocks (defaults to automatic conversion)

If your looking to make up models with the right dimensions... load up your level in MED. Then export your level to whatever program you use for modeling and go from there.

You could also use models instead of detail blocks. That's just another alternative you can use.