Designated Marksman is a valuable tool in the squad arsenal that has to alternate between ranged shots and regular infantry role. Your role as a DM is to take out those individuals that your regular infantry members cannot, as your tools are better suited for the task.
They sometimes use semi-automatic rifles, but prefer rifles that can fire in full automatic. Rifles of choice have been M16 variants primarily as SDM-R (Squad Designated Marksman Rifle) in the US Army or SAM-R (Squad Advanced Marksman Rifle) in the US Marines. They also use M14's (The Marines' DMR is a Semi-automatic only version of the M14) and several other guns depending on service branch, which nation they're from, etc.
I like the DM role. I get to make some nice long distance kills here and there, as well as work within a squad and kick the shit out of people as a team.