A few interesting posts overnight. Something from andytoshi caught my eye:
"I'm not entirely clear on what masternodes do these days, but I infer that their actions affect users' privacy, i.e. they are "confining and constraining" information. This is a cryptographic function too. Human decisions may factor into their behaviour, but they are still performing a cryptographic function; human involvement does not change this, only changes the failure modes."
This is an interesting statement and really gets to the heart of the debate.
The mixing provided by masternodes, if working as intended, is a form of one-way encryption. The process does indeed 'confine and constrain' information; information about relationships between address balances. The critical questions are around the efficacy, efficiency, permenancy and vulnerability of this process.
Efficacy: I see no reasonable argument that shows the mixing process does not produce the required result. Address balances are effectively rotated through addresses resulting in no traceable association between two given addresses.
Efficiency: The mixing process can take a while and the user experience in the wallet is not perfect, so there is room for improvement here.
Permanency: If we assume that private keys are safe then the off-chain nature of the mixing provides a permanent 'encryption' of balance movement through the public blockchain.
Vulnerability: This, it seems, is the most critical point and the subject of intense debate on this thread. Almost the entire technical argument against DASH concerns percieved fragility and centralisation of the Masternode network. Yet still, no opponent has demonstrated how any realistic adversary could hope to compromise the network, or indeed define/quantify 'compromised'.
What I see is a trade-off between utility and vulnerability. The MN network provides a two-tier architecture enabling diverse services to be developed on top of a traditional PoW cryptocurrency. Today we have the mixing service & instant transactions. Future features may include two-factor authentication, decentralised web wallets and who knows what else. It's this utility and native feature set that elevates DASH and has the potential to drive adoption. If the platform that enables these features creates vulnerabilities, then the question really is whether these vulnerabilities are realistically exploitable, by whom and for what?
So in the trade-off between utility and vulnerability there is a tipping point which defines whether DASH is fit-for-purpose. I don't see any reason why DASH is at, or anywhere near, that tipping point.
Meanwhile my technical questions remain unanswered
https://bitcointalksearch.org/topic/m.10924115