Not necessarily, it depends on where you plot the asymptotes...
Hyperbolas have a lot of use in science, and are relevant to many real-life situations. I can't think of any relevance to stocks, though.

Yes, but in a sense it's also a measure of economic activity. Transaction volume alone may often be the result of a small portion of bitcoins being moved around a lot, while BDD means a larger part of the bitcoin economy is active.
A sharp increase in BDD may indeed signal that hoarders are about to sell. But a slow increase (specifically, an increase in the ratio of BDD to transaction volume) may signal a broader awakening of the bitcoin economy.

I think it's worth even more to buy bitcoins and NOT resell them... At least for a few years.

The recent spike started shortly after the closure of Silk Road. It only intensified after the paper about selfish mining made a media splash, with headlines like "Bitcoin is broken".

There are many more examples. If there's anything I've learnt about bitcoin in the past few months, it's this: when it comes to bitcoin, all news is good news.