Even in 64-bit mode the base address for code and data segments remains 32 bits - it is only expanded to 64 bits for call gate descriptors, IDT gate descriptors

Even in 64-bit mode the base address for code and data segments remains 32 bits - it is only expanded to 64 bits for call gate descriptors, IDT gate descriptors, LDT descriptors and TSS descriptors. The base address for the FS and GS selectors can be set to a 64 bit value, but the upper 32 bits are ignored in compatibility mode (ie when a 32 bit task is running). The point about being able to implement segment-swapping is well-taken however.