Wait, what? Soft UART?! Really?! Christ on a Cracker. Most SoCs have like a ton of UARTs (the Sitara on the BBB has at least 5 full UARTs with hardware flow control for example) so I assumed the BCM would have at least two. How could they think this was a good idea? Talk about amateur hour...
For twice the price?
This was a SoC originally designed for mobile phones. Phones which usually contain more than one radio, with each radio requiring a UART for baseband or firmware programming. (Some units can do it over SPI or other high speed links now, but it wasn't common when this SoC was originally designed.)
I don't have access to this BCM's TRM, but I assume it does in fact have multiple UARTs. If I had to guess, I'd say either they originally planned to share the single UART between the BT module and header *or* it was an afterthought. Maybe they had routing issues? Or pin assignment problems. It could even be a bug in the SoC.
Whatever the reason, it seems like a glaring oversight. To be honest, I'm surprised they went with a UART based BT module. SPI is very common for BT modules. Hell, they could have even used a combo BT/WiFi chip that talks over the memory interface! (Those are expensive though, so I can understand why they didn't.)
There's a nice Maxim SPI to UART bridge chip that even has a Linux driver. I should design and sell a hat with a couple of those on it. I bet it would sell well now.