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.
The only thing that matters for a young startup is finding product-market. Parse makes that process more efficient than ever.
A person who understands backend engineering would understand the value that Parse provides.
Experienced developers are often much faster and more efficient. This is especially true if they're using a language like Python that is quite suited to prototyping, and already offers so much pre-made functionality in the form of libraries and frameworks.
Many of us have learned to be very skeptical of these services that claim to allow software systems to be build rapidly. This often only holds true in a very, very small set of cases. Otherwise, the moment you try to do something unique or unusual, you'll hit one roadblock after another. It doesn't take long before the supposed "time-saving" or "effort-saving" service has wasted more time and effort than it would have taken to develop everything from scratch.
EDIT: For the record, I really like Parse, and am a paying customer for two CRUD mainly products.