can anyone give me a reason to play those factions.
To kill those Roman dogs?

Seriously, if you don't enjoy playing barbarian factions, don't play them - it's not compulsory. I mainly enjoy Carthaginians and Dacians, but in a computer game is pretty much aesthetics that decide who plays what faction. Pretty much all of them (including 'civilised' factions like the Romans and Greeks) were very barbaric, uncivilised and brutal, much like 9/11 and Abu Ghraib more recently, but without even the modern amount of dissenters against atrocities.