You don't need Assault Rifles, just like you don't need Pistols, Shotguns or SMGs. One weapon skill is really enough, and it depends on how you wanna play really.
That said, the Pistols are overpowered mostly due to Chain Shot. And like you say, the Assault Rifles kinda lose pull because Focused Aim is just a boring special skill. But really, the Rifles are quite powerful on their own and have a huge range.
Shotguns are the most fun to use if you ask me.