x86 based systems keep getting smaller but I think there is at least another year or two before they actually end up as small and power-thrifty as ARM based Nokia ITs. So right now the NITs have a couple advantages:
1) always on with standby times of a day or so while connected to wifi
2) cost: No one can put together an Intel-based MID that can match the ~$250 price tag of the N800
3) size: I think it will be another year or two before we see any production x86 machine that's really as small as the N800/N810. We might see "pocketable" x86 MIDs this year, but I'm betting they'll be more "chunky" than the NITs.

What MIDs have over NITs:
1) raw CPU power, more memory and storage.
2) software compatibility.