Ek to yeh mechanics!
The reason they tell you to opt against it is that they might get stuck. They get stuck due to the scale and deposits developed in the cooling system pipes when you use simple tap water without coolant, the valve gets stuck, temperature rises, auto fan doesn't kicks in resulting in an overheated engine. The culprit according to them is the t-stat valve so the verdict is to always use daraaact fan which ensures that your engine will never heat up in future. -_-