The Marro Warriors are not my favorite unit.
They have below average attack and defense.
I consider them little more than cannon fodder.
There are four up sides to the Marro Warriors.
First, there are four of them.
Second, they have a ranged attack.
Third, their special ability creates more marro warriors to replace ones that have been killed in battle.
Fourth, they have above average movement.
The most obvious use for marro warriors are as a screen in front of your other forces.
They can also be a threat to lightly armored targets and their ranged weapons allow them to seek out such targets without having to chase them down.
I sometimes use them to stand on glyphs that are not guarded. Be warned that they are not able to hold such positions.