This system looks great to me if you had little to no wake/waves and were dealing primarily with tide changes.
However, if the boat takes wave impact from the side, that will get transmitted directly through the hull to the rubrail/attachment point.
Whips do a GREAT job of taking wave energy and translating it to the boat bouncing around a little bit. The only downside in my mind is that they (by design) keep the boat 18-24 inches away from the dock so there's a little bit of effort for the first person to get on the boat.
Edit: Huh, 5 year old question... well I hope he found some whips