I don't see the point when you can probaly get the real thing for cheaper (at least in my parts that is) :-)
With the verious FPGA projects I think the key point is reconfigurability, I think. How about just loading a new core when it turns out game X doesn't work with a 68030? How about loading a C64, an Atari ST, an MSX or whatever? That's what's happening over at fpgaarcade.com for example.
That, and small form factor, reliable and new hardware, low power consumption, etc.
