I understand that there were good reasons why it was done. But out-of-band control is so much nicer to work with. One channel for control, one for data.

No parsing of text. No escaping of '+++' to avoid that a '+++ATH' in the data stream closes the connection (I wonder how the download time of a file containing only '+' differs from a file with other characters).

And with increasing mobile network speeds, you really want something like DMA instead of reading individual bytes from a serial interface.