hehe, glad I can kinda chime in on this
Coming from a "tuned up" MR2, I know that stud + lug nuts setup is actually REQUIRED for upper-level racing. I heard that it actually is a safer way of securing wheels to the hub. I forget the exact details, but I heard this from a tuning guru who has participated in many auto-x competitions and some trackwork too. Perhaps this rule/requirement to have studs and lug nuts pertains only to upper-level auto-x, I'm not particularly sure, but the explanation he gave me at the time made sense. He said something about a lug bolt not being able to hold torque as well as a lug nut
As for the open lug nut, one other advantage I see is that if you have a deep enough socket/key, you could technically still secure the lug nut onto an otherwise extended stud. i.e. maybe the lug nut is recessed into the wheel bolt holes, and some of the stud protrudes out beyond the lug nut, etc.
I personally had extended Volk duralug nuts, and the reason for the extended ones was due to the fact that I put extended studs on in case I needed to use spacers, etc. and I needed to make sure that the lug nuts would fit regardless of what setup I ended up using. If I had gone with the no-spacer setup, the studs would stick out quite a bit, and with a non-extended closed lug nut, the lug may bottom out before actually securing the wheel on, etc.
This picture kinda shows what I'm talking about. The case that if the studs come out farther than the lug nut itself:
