Colour me stupid, but I fail to see how one could trip over a lifeline--unless one were in the process of going over the side already, anyway.
I agree that for anybody over a height of about three or four feet, a lifeline isn't likely to keep one from going in the drink if you're thinking about it as a kind of "safety net." To me: That is not the purpose of lifelines. To me: They're something to hang on to when there's nothing more substantial about, such as a shroud or grip rail.
Suppose you had to go forward in adverse conditions. Say you judged on-coming weather wrongly and had to get up front to take down the jib rather later than you should have. Weather and seas are so bad you can only get there safely (for some value of "safely") by crawling. You have no jacklines for the same reason that jib's still up. Wouldn't you rather have those lifelines between you and the water? I know I would.