I think the non-metal lifelines would be more comfortable to smash into in rough seas than a hard line.
Honestly, I would trust my splicing and rigging into double braid spectra than those pinch-on metal things that have slipped out on me in the past.
Another thought, was what if I had a MOB and he somehow needed us to grab him over a lifeline or was entangled? I carry a rigging knife on my belt, but not a bolt cutter... time counts in our cold waters for MOB and hypothermia sets in fast. I could easily cut through it and make the grab.