My main issue with Dash's masternodes is that they're so expensive to participate in. I know there are systems setup to pay for "shares" of a Dash masternode, but those are completely dependent on you trusting the operator to pay you. There is nothing stopping them from just taking your payment a keeping what
"But Casper's Validators are said to require 1000 ETH to run! That's over $300k USD! That's about the same as a Dash masternode!" Not an awful argument, but since Casper is done almost entirely on smart contracts, then it can easily be made trustless by having functionality implemented in smart contracts that protects the users that are pooling there ETH to run a validator.
For your point on "transaction locking", I don't really think it's necessary. Dash's blocktime is ~2.5 minutes, so the "instant transactions" are more useful. Ethereum's 14 second blocktime makes it much less necessary. Instant transactions can also be implemented by solutions like state channels, i.e the Raiden network.