i've been discussing stuff with aguire last week and i just tend to slip into that now and again...
basically, in order for quake to do collision detection for players and monsters, qbsp builds invisible "hulls" which are actually just extruded versions of the original map. the game then performs point entity collision detection (which is faster than boundingbox collision detection and was what was needed to make the game work back when it was made) like with grenades and rockets, but against the extruded version of the map.
but because qbsp isn't perfect, the extrusion process sometimes makes odd geometry like troughs betweens brushes.
you can see, i've traced the edges in black. this is the normal, visible hull that is usually in q1 maps.
hull1 (extruded version made visible with aguire's qbsp):
the red lines denote where the edges are in the normal visible hull. this is hull1 used for collision on player sized monsters. (and the player...)
you can see how the brushes have been pushed outward-- extruded and has formed troughs on the edges.
in fact, i noticed later that a more visible example is on the edge which is behind the text in the second shot. :P you can see the trough shape that qbsp made more clearly there.
anyway, this is why making terrain with monsters on it would be difficult. the monsters would have a hard time moving around because they'd be getting stuck on these troughs.
sorry about the quality of the pics, btw...
hope that explains it. :)