Its just not that hard to do a 'serviceable' back-end behind a simple rest API. I mean how many hours is that really going to take you, especially when you consider in the case of parse users they already know what data entities their application uses? A REST API for a dozen or so already defined entities, maybe a weeks worth of work? And then look at what it buys you, actual control over your data, ability to move from parse and pursue cost cutting alternatives, ability to do in depth analysis of your data...
I absolutely think Parse has great value, but when the statement is made that in general it might replace back end engineering for startups... its just wow man. This is precisely why a person who understands the server side of a client server application should in general always be involved if the business is based around client server applications.