I use living armor in one of my campaigns - thought that is a blend of mollusc, human, and demon, so quite unlike yours.
Good side effects of the micro-beings are disposal of waste and body odor.
They might, as they are telepathically linked to the wearer, work as a sixth sense: percieving waves (vibrations, air pressure generated by movign objects, etc.), or even notify him if something moves behind him.
They could produce perfume, or a myriad of other useful things - skin lotion, healing potion around wounds (or just help stop bleeding), or stimulate musles with electric impulses while you sleep or just rest, so that they get constant exercise.
You could use them to mark a person or object, to find it later. Some armors might work as a weapon, producing poison that is harmless to you, or they migth infect a foe and cause him harm.