I found Dynmap a bit spoiling, some times people want to be far away from others or to hide their creations- Having a Dynmap by default shows all player locations and anything they've made on the map.
Example: People wandering miles away from spawn to prevent their homes / Towns / Creations from getting griefed en masse; With a Dynmap? They now know where every point of interest is.
Uses of it though is to see stuff on the map, see where people are located, and find where spotted biomes are. Given the previous server I was on allowed for PvP, the map was mostly used to stalking, and I killed many a stalker.