Dual clutch trannies are a bit heavier, relatively expensive, and shift quickly. Also, they have lower driveline parasitic loss comparable to manuals, better than hydraulic autos.
The newer autos from Nissan, BMW, and others have locking torque converters which largely eliminate the driveline loss advantage. They also shift faster than before, but still not quite as fast as dual clutch.
I'm trying to be objective here and not let the fact that I have a 7AT color my perception, but I think Nissan is being pragmatic. A dual clutch would make a Z (a little bit) heavier and more expensive, while improving shift speed a small amount. There's also some prestige in having dual-clutch. This makes sense in a $75k car like the GTR, but not so much in a $35k Z. Nissan could have put a dual clutch in the 370z, but they went with the 7AT. Their reasoning isn't likely to change until the next rev.
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