With bluetooth you need to consider radio interference between the sensor and the main device. In my experience that increases complexity of the sensor, and is something you largely can avoid with a module that has a physical interface to the main device.
I'm hoping that project Ara will become the perfect test bed for sensor module development. Useful after the initial validation stage of experimental stuff on a breadboard but before the stage where you have established enough customers that you can afford to develop a completely integrated and standalone IOT device.
There are lots of mobile devices specifically designed for industrial and commercial use. Usually they are much more expensive, but add few additional components. It would be useful to be able to build up your own devices from standalone components. But I doubt it is really in the interest of the manufacturers who want to be able to offer a complete "solution".
To put it another way, if you were running a medical devices company with limited resources, would you bet on this or bluetooth?