In Colorado, I plant to four or five feet, which I consider to be relatively close vine spacing.
Vines are cheap here, direct rooted at 0.65 a pop in quantity and you can get 1/2" bamboo for .08 each so cost is not much of an issue. The advantage is you can fill the trellis faster and
produce a larger crop earlier.
Numerous cold injury or vineyard management problems can cause an unpredictable number of vines to do poorly. This is less of a problem with close spacing; the canes of the stronger
vines are simply extended over the trellis, taking up space of the ones that do poorly. Or maybe all the vines do more poorly than expected. I see close spacing as a kind of an
insurance policy, kind of like double trunk training, only better.
If you double trunk train, the vine reaches production height slower than with single trunks.
Planting more vines with single trunks gets you the insurance benefit of double trunk training but with faster growth. If you end up getting too much vigor you can go to a split canopy and
increase your yield. Or you can always rely on a cover crop to reduce the vigor. Close spacing at the price we pay for it seems to me to be a no lose situation. Now if I were
paying $3 for a grafted vine, I'd have to think twice ...