I find that rehydrating with some wort or must between 2-5 hours prior greatly increases my fermentation speed (I can have a beer made grain to keg in about 5 days vs. 7-8 days). I also do not do anything to make sure that it is sterile; I sanitize everything with StarSan, fill a container with 2 ounces wort and 5 ounces filtered water to cool it, add the yeast, and cover with aluminum foil.
I also think that the dehydrated yeast is highly condensed in its dry form; by having the yeast rehydrate with water, instead of a high sugar and nutrient liquid like wort (which will make the liquid more dense), it will allow the yeast to expand easier as well as reduce the chance of shocking the yeasts’ membranes with an immediate concentration of sugar, potentially stressing and scarring the budding process.
The likelihood of an infection occurring in the starter after the yeast is added is very small, the yeast are crowding that container as it is, there isn’t a foothold for other bacteria to colonize inside of it.
I can see the conversation going both ways for time and results, I think that if you have the time to rehydrate, it can be helpful, especially in nutrient poor liquids, like wine musts.