Right, this is a centralized approach... a central server can deanonymize people. There may be many of these servers, but you're still trusting them to not be bad.  It may be acceptable— it's probably better than nothing at all.  But things like this is precisely what Ozziecoin is slamming.  Ironically, because the CJ thread post 5 describes how you can deal with the dos attacks while actually being private for everyone:  If the transaction fails, everyone deanonymizes their attempt, and anyone who fails to deanonymize (or is directly shown to be the party refusing to sign) is banned. It's a PITA to actually implement, I agree.

PS, masternode centralization in the future doesn't cause any problems for darkcoin. I have 2 possible solutions to evaluate for V2 of darksend (ring signatures and encrypted system where the users themselves do the joining relayed through the masternodes.) . Both of these make the masternodes unaware of who is sending money to whom, so centralization isn't an issue at that point.