We sailed a Martin 242 (relatively hi perf day racer) without lifelines for nearly 15 years. We had no issues other than obviously having to be a bit more cautious moving about. I know many who refer to the so-called "lifelines" as 'deathlines' and they believe that you're far more likely to trip over them and go overboard than not.
That may be a bit of an exaggeration, but in reality on most smaller boats any lifelines that 'look' right for the boat size will tend to be rather low, e.g. knee height and unless you're rolling on your back they are not likely to prevent a fall overboard. They do, however, provide a last ditch 'grab point' that can help you stay on (or alongside) the boat. And that's as long as they don't fail when put to the test.
Stanchions are notorious for being under-engineered, poorly supported/backed and probably the most common cause of deck leaks, core and skin delamination due to water intrusion. Any force applied to the upper end of the stanchion (or upper lifeline) has a long lever arm and really stresses the stanchion base. Stress cracks in this area are commonplace, even if leakage or delam is not an issue.
Advantages of having some kind of lifelines is that they do tend to try to keep hanked-on sails on deck when dropped, and of course when properly fitted with webbing or lacing they are indispensable when sailing with children.
...... They are required for offshore racing.
True... we once sailed on a sistership to our own M242 in a distance race that required lifelines.. it was awful, seating positions in the cockpit were really compromised from normal... but 'thems were the rules'....
EDIT... I should qualify the above by saying that we have, and have no intention of removing, the lifelines on our current boat!