I disagree with Floridatennisdude's statement that ladders that let players play as much or as little as they want are inferior. The relative merits of various approaches depend on what the goal of the ladder is.
If the goal of the ladder is to encourage people to play tennis and offer them opportunities to do so, allowing them the chance to choose how often to play lets people with time constraints or business travel or the need for a babysitter to participate.
If your goal is to prove once and for all which player is the best 4.0 player in your area, he might be right.
A ladder can account for varying amounts of playing time by giving more points for beating a player high above you.