Stage feeding Honey is generally done when one desires a High alcohol volume. Often times too high of a Specific Gravity (SG) will cause the yeast to stall. If you stage or step in honey and know exactly how much honey of a given value you intend to use and the SG produced by said volume then, yes The alcohol content is predictable. However often the closer to the end of the cycle the smaller the amount of honey is added as at some point the yeast will quit due to exceeding it's alcohol tolerance. Even though yeast manufactures offer a tolerance range it is not always exact and can very considerably. So the trick is to add honey in a manner that maximum alcohol is produced, while still achieving dryness here is where the calculations come into play. When making Meads within acceptable levels for the yeast there is no real benefit to stepping in the honey.
Slow ferments when making mead helps maintain the individual flavor profile of the honey. Fast violent ferments transition co2 and burn Oxygen as well as devour the nutrients the yeast requires. Although we often talk about oxidation of mead and how it can damage the meads flavor and color. During the onset of fermentation Oxygen is necessary for the yeast to develop. A slow development of the yeast places less demand on the nutrients required for it to ferment the sugars properly. Fast ferments can cause the yeast to become starved for nitrogen and produce Hydrogen sulfide which can give the a burnt rubber taste and smell.