The Doom Bat is a particularly odious form of giant bat. They feed upon savagery and are adept at hiding in underground caverns. Their horrible shriek induces fear and panic in those who hear it. Doom Bats strike their prey with a deadly venomous bite.
Some beings have been able to capitalize on these creatures' slight degree of intelligence and train them to fight in combat. Orc Shamans use them as spies and messengers, and it is thought they are possessed by the most honored Orc dead, given one last chance to wreak havoc before they retire from the world.
Their ability to fly and navigate in the dark makes the Doom Bat an excellent scout.
Doom Bats are useful only in certain situations and not on a large proportion of maps. Their usefulness is highly dependent on a map having an underground layer and ideally a depths layer as well. In a map without any Level 4 cities, their ability to fly may be useful as well in lieu of having Red Dragons, depending on the layout of the map.
When used underground they are effective for scouting the map and performing small raids on exposed cities and resources, but their very low combat stats and Health means they shouldn't be used for large-scale fights or against strong units. They lack a projectile as well, which means they have to engage enemies physically. Their underground concealment ability makes avoiding combat easier, and can allow you to sneakily steal your enemy's structures if you play it carefully.
Essentially any Level 3 or 4 unit in the game with either a projectile or the ability to fly can defeat a Doom Bat in combat quite easily, and even a small army of 2 to 3 basic archers can take one out. Countering them in combat isn't the issue, but rather locating them underground. Having a hero with true seeing is important for any race besides the Highmen, whose units have that ability inherently.