Calling the masternode reward "ROI" is therefore a misnomer. It is not a 6% "return" on anything. It's just 6% of the collateral but that's not what you invested, it's what you purchased WITH your investment.
I invested in Dash because of its masternode rewards and its MN payment schedule (going from 30% to 60% of the blockreward in a short period of time.
I therefore think it is perfectly reasonably to call it ROI.