It may have made more sense in his time -- when grievous injuries and deaths were common in the workplace, when striking workers had a real risk of getting shot, and when hours and pay kept many close to bare subsistence -- but it largely doesn't now, at least in the U.S. Tell someone you're a wage slave in the U.S. and unless they already agree with you they're going to think it's an obscene comparison. Now you have two problems: they still don't agree with you, and you have to argue the comparison before you even get to your larger point.

It's not hard to describe how exploitative capitalism is. We don't need to exaggerate for effect, and doing so just gives people an easy out if they want to write you off.