Think of it like a treasure hunt, while a map may not pinpoint where each bit of treasure is, having a map in the first place provides an advantage over someone who doesn't have one. You can never really get lost when you have a map, and it improves coordination as to where you are in the "arena".
It also pretty much makes in-game maps defunct, as you can already see where everything is from the same overhead perspective, so where players have achieved a legitimate advantage in-game, minimaps devoid such, as where the legitimate player will have to choose between their map and anything else in their inventory, a minimap user can have anything equipped as well as their map.
Some minimap mods also allow for the map to be saved as an image, this allows players to edit them outside of the mod itself, to mark where chests are, and what route they will take, they can then use this while playing, this provides an extreme disadvantage to all users, minimap or not, and is absolutely undetectable.
Just a few reasons why I think minimaps need to remain illegal on the servers, I'm sure if I were to sit down and think for a while, I could come up with many more reasons than these.