The obvious drawback would be that such device connected to charger without current limiter would burn it.
Then again I just burned a charger that had multiple sockets by connecting Samsung Galaxy Note 3 and iPad2 to it. It advertised via data lines capacity for 2A charging on all five USB sockets, but had total capacity of just 2.5 A.
if lithium batteries weren't so problematic when overcharged you could float-charge everything pretty efficiently, then you wouldn't even need a management system.
Our current way of doing things is probably here to stay, though.
A data channel isn't a data channel. USB is designed to be widely used by many industries, as such the standard has provisions for many use-cases. A battery data channel is only for the charging equipment to communicate with the battery's chipset a limited amount of parameters. The data being transferred is incredibly limited, and can thus be sanitized easier when compared to USB.
A protocol designed in such a way would also be easier to test, as the scope of vulnerabilities would be much more limited than a general purpose data channel.
Regardless, it was merely a possible answer to
> If you disconnect the data pins, how can any device (apple or anyone else) get more?"
and really wasn't meant to be a valid product or concept. Just a fleeting thought.