Personally, I feel that moving away from Arduino / Wiring compatibility,
Right, because this is what STM32duino stands for: Compatibility with the Arduino API.
Personally I think, leaflabs left a big gap in the libmaple: strange codings, like 3-4 different "spi.h" with the same name,...unfinished things like I2S, no real USB stuff like audio device, mass storage... and so on. Another point is, that libmaple has only a "real" support for STM32F1 series.
So my only real important question for moving to a new core:
When it's done, is it easier to set up different series and models (like the new STM32F7) and reconcile them? So this would be the major benefit. We wont have to take care what's on "layer 0" because i.e. cubemx+hal would do the job for us. (updates, bugfixes, new models/series..)
Another benefit could be, that's more easier to convert all the other examples for STM32 found on the web, so STM32duino could be a "glue" for Arduino and the rest of the STM32 world.