It's my contention that this is on purpose. Its very much in the design to leave things this way and let table rulings determine the outcome.
They've come right out and said "Rulings not Rules," yes. There's no point putting the kind of 3.x RAW-uber-alles stridency into a 5e discussion like this. Whatever the rules literally say, and however they might be interpreted, when you ask your DM "I'm preparing my spells, can I just do some of them, and finish the rest later?" all that matters is how he rules, at that time.
For DMs who like to think about the repercussions of their rulings (and that's certainly not all of us), this thread may have given a few things to think about...
For instance, spell preparation is, mechanically, a way of increasing the class's flexibility, it's not a restriction. A Cleric isn't 'limited' to changing his prepared spells after a long rest, he's enabled to do so. A sorcerer or warlock or EK only gets to add to his spells as he levels, he can't just change 'em up after a night's sleep. Letting a caster spread out preparation choices over the day just gives him that much more flexibility, and 5e neo-Vancian casters are already the most flexible casters have ever been in the history of the game, combining as they do the advantages of traditional Vancian & 3.5 Spontaneous casting. So the question is really whether an additional soupçon of flexibility would break the neo-Vancian casters to a meaningfully greater degree.
Personally, I don't think so. The neo-Vancian system removes a lot of the potential frustration and paralysis from playing a prepped caster, and this particular Ruling just takes it a little further, which would be helpful to new players trying to cope with such characters, and wouldn't render them that much more abuseable in the hands of system masters.