what exactly are multiple rom buses good for? as far as I knew, the NES had only one...
The NES had 2 ROM buses, one for graphics, one for program code. The pluses of having multiple ROM buses is that the overall speed of the system is greatly increased, as the CPU doesn't need to load graphics from ROM, [decompress it] and display it on-screen, resulting in a delay. Nor would the CPU, graphics, and audio HW have to share the same address bus, resulting in a delay as each hardware unit gets its slice of clock time.
The NES was pretty quick at animating its entire screen in certain ways because it could rely on the MMC/mapper to do graphics bank swapping. The Neo-Geo similarly has separate buses for its graphics ROMs, sample ROMs, so each unit can function mostly independently without lagging the CPU.