In the US this airspace structure is designed to allow IFR aircraft to get off the ground from a non-tower airport (so they are IFR in G without ATC contact and then make contact when they are in the air) or in relatively flat terrain to allow for an area for VMC flight in poor viz (i.e. clear of cloud and in sight of surface, aka scud running) or for a corridor under controlled airspace (also normally in relatively flat terrain).
For really undulating terrain the gap under CAS isn't really usable.
In the case of a mountain hole (say a plain 20 miles in diameter at 5000 ft with peaks to 10000) you could get potentially have an IFR approach into an airport there, as such 1500 AGL would be 6500 in the plain and then follow the mountains up.