If Dash's instant transactions work as dinofelis says they do, then they are not worth the digits they are stored in. Masternodes are really nothing more than a taxation method on miners earnings for providing services of dubious quality.
Well, from the paper that describes this way of working:
https://www.dash.org/instantx/Locking messages will propagate across the whole Darkcoin network and reach all clients. Once the lock has reached everyone, a set of deterministically selected masternodes will form a consensus. Next, upon a successful consensus, a message will be broadcasted across the network and at this point all clients will respect the lock on the funds
--> the part in bold is a clear impossibility in a P2P network and is essentially the fundamental problem of consensus making.
and
By utilizing the masternode network, we can gain a degree of certainty that the transaction in question is valid and will be accepted into the blockchain after that. Immediately after the propagation of a lock, the selected masternodes will begin to vote on the validity of the transaction lock. If consensus is reached on a lock by the Masternode network, all conflicting transactions would be rejected thereafter, unless they matched the exact transaction ID of the lock in place. Clients would be tasked with clearing out conflicting locks and possibly reversing attacker transactions. This would only happen in a case where an attacker submitted multiple locks to the network at once and the network formed consensus on one but not the other. If no consensus is reached, standard confirmation will be required to assure that a transaction is valid.
3.2 Election Algorithm and Voting
A special deterministic algorithm is used to determine a pseudo-random ordering of the masternodes. By using the hash from the proof-of-work for each block, security of this functionality will be provided by the mining network. Pseudo Code, for selecting a masternode:
Essentially, this mechanism proposes that once consensus is reached, it can reach consensus
If it were possible to ensure that "all nodes received the message", there would never ever be any consensus problem. In other words, instant pay works well, if there is no double spend attempt.
After that, the miners still include what they want, if I understand well. However, most probably they are asked to include instant-pay confirmed transactions first.
It is important to realize the circular argument in this, because it can sound credible. The whole, and the SINGLE difficulty in setting up a crypto coin, and the exact reason why Satoshi invented Proof of Work and the Block chain, is that it is essentially impossible to:
1) make sure that EVERYONE on the network got the transaction
2) that the order in which two contradictory transactions are received on the network is the same for everyone
If one could make this happen, crypto would be MUCH MUCH simpler. There wouldn't be any mining, blocks or whatever. Indeed, every node on the network would receive ALL transactions in order, and only accept the first one if there's a double spend attempt. There wouldn't be any need to put these transactions in blocks, to mine them, to "secure" them with PoW or anything of the kind. That whole circus only serves ONE SINGLE PURPOSE: deciding, once and for all, in what order the propagated transactions should be processed, and picking (at random) one of the conflicting ones.
If you can SOLVE this problem in one way or another (like DASH pretends, with its propagating "locks"), then there's no need for blockchains, mining or any other securing. Propagating the lock, or propagating the transaction, is the same. The first one should be locking out the second one, and it is exactly because this cannot be rigorously imposed, that all the hassle with block chains has been invented. So if you can propagate the LOCKS correctly, you could have propagated the transactions correctly. And no more mining, blocks and whatever.
So instant pay solves the problem, when the problem didn't need to be solved.