I always keep 4 independent address spaces :

1. Program memory (read only, high bandwidth)
2. Data memory  (read and write, as well as scrambled access orders)
3. I/O and Special registers (serialising, one access at a time)
4. Registers (can sustain multiple simultaneous reads & writes)

Each is optimised for a specific purpose and things get nasty when they are mixed.

• mix data and program memory, and you get a lot of security troubles.
• mix data and special memory-mapped registers, and you kill your system that must recognise IO regions and addresses, as well as ordering and write-through/writeback/grouped/caching attributes
• mix data ram and registers and it becomes ridiculously slow (TI tried this last with the TMS9900 and you see what happened)
• ... you get the idea.