Maybe a drawback is that it gives the all-sight to ennemy too...
Exactly. I like to play with spotting hidden units activated, since the exploit is far too inviting in a hard battle. Also, I think melee battle are not about reconnaisance, as is Panzer General for example.
Recon was always needed, long before Panzer General and the battles of WW2 it simulates.
There is a reason that light Cavalry has a large spotting range and some units (Juns) have a special ability that allows them to see even larger parts of the map.
When you play with enemy units hidden in the fog of war, then spotting them is essential to use your own troops to avoid running into an ambush (if you move a unit into the fog of war and there is an enemy between your starting point and your target point then the enemy will ambush you and receive a bonus in that fight).
Or when you advance too far into the fog of war and notice only then that there are several enemy units that can attack you in the next turn while your other units are still lagging behind... Recon can prevent that.
A historic example where a roman general was too lazy to do proper recon was this one: https://en.wikipedia.org/wiki/Battle_of_Lake_Trasimene
While switching "spot hidden units" on allows the enemy to spot you too, it does not memorize that. So for an easy game you can switch it on, see the enemy units and plan accordingly, then switch it off and the AI makes it’s move without knowing your positions.