There are some applications where actually boiling the milk is needed, for example when you want to precipitate out some of the proteins - they build that fuzzy film on the bottom and the skin on top during cooling. In other cases, it is about sterilization - for example for yogurt, you keep the milk boiling for longer time, and it is almost impossible to keep it at just under boiling, unless you make a sophisticated thermoregulated setup and take the extra time needed to heat it slowly so you don't overshoot.
You also have to consider whether the temperature is needed by ingredients other than the milk and the starch. For example, if you are making a pudding with both starch and yolks, you have to hold it a couple of minutes at boiling to deactivate the amylase.
For milk rice, you don't strictly need boiling, but you have to get really close. Different starches thicken at different temperatures, but practivally all I have read about are between 94 and 100 Celsius. So if your starch hits the sweet spot at 97.2 C, then just watching it and noticing that it is very close to a boil is not of much help. First, you run the risk of stopping too early, which will ruin the milk rice(as opposed to stopping after it boils, which will result in good milk rice). Second, the last few degrees of milk boiling go so fast, you may not manage to remove it from the heat before boiling anyway.
If you know the exact temperature for the starch you are using, you could in principle get away with making milk rice a couple of degrees lower. It is rather impractical, but possible.