The Medium Tank is one of the most versatile units in most battles, but isn't effective in doing one particular task. The Medium Tank is a very flexible unit on the battlefield, as it is able to engage infantry more effectively than the Heavy Tank, and can also combat ground vehicles. But it isn't effective at either task; the Medium Tank can often miss using it's Anti-Infantry ability, and a Medium Tank cannot withstand attacks from Heavy Tanks.
The Medium Tank can be used as an all-around units, when the enemy seems to be using a mix of infantry and vehicles. It can also use it's speed to its advantage, as it is faster than a Heavy Tank and can be used to flank the enemy or hunt down enemy Anti-Aircraft units.
The Medium Tank isn't all that good against Armor units. A Heavy Tank will rip one to shreds, and the Light Tank's and Armored Transport's special ability can do more damage than the Medium Tank's main armament. But the 2 latter vehicles have a disadvantage of not being able to fire their missiles all the time, and the Medium Tanks gun can do a lot of damage against both.
Though it's armament is not a major threat, it's speed is. The Medium Tank can catch up real quick with AA units, making it a dangerous unit.
The Medium Tank is the bane of infantry units. A large group of infantry units are in immediate danger if caught in the open when Medium Tanks are around.