Just because a roll cloud isn't linked to a storm doesn't mean it should be ignored. This one drifted over southwestern Ohio in 2006, for example, 5 to 10 miles ahead of a severe storm system that was blowing in from Indiana. The spooky scene gave residents an early warning.
The National Weather Service notes that when an arcus cloud outruns its thunderstorm like this, it may be "a sign the storm is losing its potential to produce severe damaging winds." Still, that's not always the case, and it's often a good idea to take such foreboding skies seriously.