"Notwithstanding the fact that the currency peg is very political, are there any compelling reasons why the HKD should not be "unpegged" from the USD, particularly in view of expected further depreciation of the greenback?"
That's an excellent question! To answer it, first we need to take a slight detour, into the world of currency unions. A currency union is a form of monetary policy where two or more countries use the same money- for example, the Euro area, or Ecuador and the US- who both use USD. There is a substantial amount of evidence that countries in currency unions benefit economically from large increases in trade, investment flows, and output (see the links on this page put together by Andy Rose at UC Berkeley, and my own modest contribution published in Pacific Economic Review that you can view here).
So currency unions are good- but what's that got to do with Hong Kong, with doesn't have a currency union, but a currency board? It's hard to say anything definitive, as there just aren't enough cases of currency boards to undertake the kind of empirical studies linked to above. But I think it is a reasonable conjecture that the same benefits that accrue to currency unions also accrue to currency boards. Both fix the exchange rate in a way that is politically costly to reverse; the main difference between them is that in one case, the countries retain different notes and coins from each other, and in the other case they do not. If my conjecture is correct, Hong Kong benefits from higher capital flows (important for the establishment of the growing international finance centre here), higher trade flows (one of the corner stones of the Hong Kong economy), and higher economic growth.
That doesn't mean that a currency board is without costs. Since the exchange rate cannot adjust to absorb shocks, other variables do instead- including output and unemployment. Our business cycles may be more volatile, but that may be a price that is worth paying.