The built in cadence sensors within the InfoCrank power meter operate using extremely fast acting reed switches. The magnetic sensor is more accurate, consumes less power, and has faster start up and response times than other methods of measuring cadence.
Alternative cadence measuring methods such as accelerometers would not give the response rate required for functionality of the power meter. A cadence magnet is still the most accurate measuring method, so while an accelerometer would be accurate on average, it may not be as accurate per pedal stroke.
See the attached presentation to see examples of accelerometers vs cadence magnets.
Did you know you can now use InfoCrank without cadence magnets? Have look at our magnet free RevUp firmware FAQ http://support.vervecycling.com/support/solutions/folders/1000224997