Some people would prefer not to pay taxes if given the choice, but you are correct with this thinking and that is why the premium is most likely a reflection of the tax rate of it's buyers with a little boost for those who are ignorant and hit buy without researching. This is something I wasn't expecting, and if it holds true for COIN, things could get interesting, particularly for the ones who bought all the BTC for the fund. It's starting to make sense now why the Winks didn't bat an eyelash at selling $1,000 BTC. They're going to make an absolute killing on premiums and maintenance fees. It's actually scary how rich they would become if BTC is truly successful in the long term.
If COIN can achieve top brand status, they could very well make a killing. I'm not sure if it'll be socially on par with building a Facebook but it could end up more profitable in the long-term. The tax thing is likely a huge motivator. If you're paying capital gains of 15% or more if short term or in a greedy state, the price doesn't have to rise that much (in Bitcoin historical terms) for it to end up profitable versus buying BTC.
The trading experience is unique or maybe it isn't but others can confirm if they're seeing the same thing. My first trade went for more than I asked for though it was a small quantity limit order. The second was big enough but doesn't seem to have shown up all that clearly in the volume. Though I figured out who my orders would show up as, it seems they are only provided in aggregate with whoever is trying to trade through them and if I'm not the best bid or ask, my order is hidden.