Yeah I suppose the disadvantages outweigh the advantages. However, I think you should mask unexplored map areas with black (i.e. can't see terrain, props and trees) fog of war. This is to encourage new players, who haven't played the map before to explore and discover the areas instead of being able to "look forward" and see where everything is in advance. This way, new players will be more excited when they see the cool new area they advance to like the Mytargas city and forest and, later, the foothills etc.
Of course this will not work with games where high lvl players and low lvl players play separately, but either that can be fixed with no share vision between people who are 5 levels or more apart (and 2nd condition checking whether they are within 1800 range of player hero, in which case share vision is enabled anyway) or just accept it as it is. Also disable See all Visibility and Iseedeadpeople cheat for proper effect.
Following this idea further, quests that focus on exploring certain areas could be added. Like "Find the ogre cave", Water Cave, or whatever. Since you already have such quests in game I suppose it wouldn't be a problem to add.
On another note, sorry for slightly hijacking this thread, but I believe the overall map suggestion fits it.