I think it's worthwhile in these times when Hong Kong is only ever talked about as a millstone around the neck of the PRC government to reflect on the value that Beijing and the PRC elite as a whole still garner from Hong Kong being (at least in name, if increasingly less so in fact) a semi-autonomous part of the People's Republic of China. Ho-fung Hung over at China File does a good job of running down the concrete benefits of the H.K. S.A.R. that the CCP leadership still enjoys:
The crux of the issue is that Beijing still desperately needs Hong Kong as a front man to do lots of things. It uses Hong-Kong-registered entities to conduct sensitive deals such as the purchase of former Soviet carriers, the digging up of a canal in Nicaragua, and the hiring of a former Blackwater CEO to assemble team of mercenaries to protect China’s investment in Africa. It offers Hong Kong as a safe haven for its notorious friends like Mugabe to store their private wealth (Mugabe’s daughter graduated recently from the City University of Hong Kong, his wife is spotted regularly in the most luxurious shopping malls in Hong Kong, and the family owned a villa in the Jackie Chan Castle in Hong Kong). Hong Kong is also an important channel through which the princelings move their assets to the U.S. (After Bo Xilai’s downfall, it was disclosed that he maintained vast property in Hong Kong and his wife, Gu Kailai, has a Hong Kong identity card). And above all, Beijing needs Hong Kong’s autonomous status for developing a RMB offshore market, internationalizing the currency without liberalizing China’s capital account. All these require foreign countries, most importantly the U.S., to treat Hong Kong as a de facto independent entity and treat the migrants, goods, and capital from Hong Kong as different from those coming from mainland China.Whilst the propaganda value of Hong Kong's return to the motherland may well have been long since used up on the mainland, and the essential failure of the "One Country, Two Systems" in Hong Kong means that the idea of attracting the Taiwanese into Beijing's orbit by offering it is now a dead letter, the value of Hong Kong to the CCP government is not so different to what it was during the 60's when it was the small opening through which China communicated with the outside world. Whilst nowadays the PRC's access to world markets is immeasurably greater than it was in the 1960's, there are still many ways in which it, as a still-developing country, benefits from the existence of a first-world economy over which it has easy access and control.
In last year's white paper on "One Country, Two Systems" the Chinese government explicitly stated that Hong Kong's autonomy came "solely from the authorization by the central leadership" and was subject to the central leadership's authorisation. It then went on to state that Hong Kong would only keep its autonomy so long as it "fully [respected] the socialist system practiced on the mainland". This was a clear threat that the PRC government did not see itself restricted by the Sino-British agreement and could withdraw Hong Kong's autonomy at any time if it felt that it was a threat to the PRC's rule - and given how widely the PRC government defines what threatens it in other areas, this could be at any point.
However, this threat seems pretty empty when you consider just how much especially the PRC elite, with their HK bank accounts, HK property, HK ID cards and so-forth actually benefit from the HK SAR's continued existence.
Whilst the PRC does have a far more pliable entity for these purposes in the form of Macau, Macau never enjoyed the credibility of Hong Kong as a financial centre. Moreover, much of the credibility that Macau did have pre-1999 has been lost amidst scandals surrounding the activities of organisations like Banco Delta Asia. Reducing Hong Kong to the same pliant state that Macau is in would likely lead to a similar loss of credibility, and hence usefulness.
[Picture: The future Chinese aircraft carrier Liaoning is towed through the Bosphorous, supposedly on its way to Macau to become a floating casino after being purchased by a Hong Kong company that instead appears to have been a front for the People's Liberation Army. Via Wiki]