Wally,

On a small boat they really aren't lifelines as such but more of
outboard boundary markers.

On most boats MOB's are usually miss steps because people lose track of
where they are, while doing something else besides watching for when
they run out of boat. I don't know if the low lifeline is more of a
triping hazard or slipping off without a line is greater?

Just thougth I'd mention that the line reminds you of where you're
stepping

Ole Thom