Are you sure you mean the masternodes would perform mining in the form of PoW, or do you mean they are simply responsible for consensus and signing blocks as in a type of PoS scheme? I can understand the latter but not necessarily the former.Centralised pools are a complete perversion of Satoshi's intent. The sooner they are eradicated the better. And if Darkcoin comes up with a viable means of doing so (we could have have one - decentralised mining via Masternodes) we'll be a hell of a lot better off in all ways.
It would also be excellent PR: Darkcoin solves the 51% problem that hangs like the Sword of Damocles over the head of every other cryptocurrency.
I started a thread to try and capture this topic here