I spoke with a PGMOL referee recently who informed me a bit about this a few weeks back.
Basically TAG Heuer came to them and said 'What would you like'. The PGMOL referees basically said a stop and a start function, but TAG Heuer were insistent on going overboard. The watches have alsorts of apps built into them, Google Maps etc. When they started sticking all this in the PGMOL referees went with it and asked "Can it have a heart rate monitor built in" and "Can it have goal-line technology built in." Both responses were no but that they'll work on it for next season. Basically, due to them not wanting to wear a shed load of different watches, they've stopped wearing the wrist HR monitors. The watch has a load of stuff they don't want, and doesn't have a load of stuff they want. Hopefully they stick to their plans for next season and integrate HR monitoring and GLT.