[WraithLord]

Usually w/o diplo one concentrates on deterrence to avoid wars. It may actually be more reliable then NAPs. As I have seen once and again that in diplo games you'd get nations dog-piling the leader while in RAND that's a rare occurrence and even when it happens you'd see 2-3 smaller nations banding as opposed to much more in diplo games. Also, when you're smaller and weaker in diplo games you may easily get to end game while in RAND it will all be over much sooner (so the whole game is much shorter as well).

I speak from personal experience of playing both game types. For me removing diplo considerably shortens turn processing.
EDIT: Again, I'm not saying diplo is bad, not fun or what not. It's perfectly viable and I'll surely play diplo game in the future. However I think that games w/o diplo require considerably less time investment.