Problems I can see:

If the sensor is on the robot:
field has transmitter
-can't have IR reflective material for the walls (multiple interfering signals)
-fairly expensive if its on game pieces that get destroyed
player has transmitter
-players transmitting at the same time interfere with each other
-2 teams with the same signal for different things can mess with each other

If the sensor is on the field:
Robot has transmitter
-need to be fairly accurate with the transmitter
-2 robots transmitting at the same time = nothing happens (interfere)
player has transmitter
-isn't there an easier way than IR, like just 4 buttons?

If the sensor is at the player station:
field has transmitter
-why use IR for this?
robot has transmitter
-robot can transmit data through the radio, why use IR?