In most cases, you won't need another temperature probe.
The second sensor could monitor temperature just after the element and have a high value switch off function. If the wort exceeds a certain temperature, the controller would lower the element temperature, cycle it or shut it off entirely.
In most RIMS systems, the flow rate seldom lets the wort temperature exceed 10oF above the incoming wort temp. Yet, if you have issues with stuck mashes then slowing the flow is the best answer and that could lead to temperatures exceeding 175oF. In that case, you'd want to know when you're near temperatures that denature enzymes.