The SensorTag does some of the processing of the data before sending it to the phone. For an application like a pedometer it is no problem to do the processing on the CC2541, for more complex applications it may make sense to use an external MCU and use the CC2541 as a network processor.
For a similar use case, we are using the same MCU on the advanced remote(http://www.ti.com/tool/cc2533dk-rf4ce-ba) In this applications the MCU is reading 6-axis motion sensors and performing sensor fusion algorithms on-chip before sending processed data to wirelessly.