Draws will be ignored. A ladder lacks all hints on how large the difference in abilities between two players really is, or any other linear scale of reference to compare players, thus making it hard to determine a worthwhile challenge. There is also no straightforward way two players could have equal rank. The ladder has a few things going for it, though. The history of the Elo system teaches us that simplicity is an important property.
Simple solutions are good solutions.
The less detailed a ranking scheme, the fewer free parameters, the less argument between players. Each actual match is a sample , a data point during our quest for the true player hierarchy. It might be necessary to introduce a penalty for ignoring challenges, as well as rematch regulations and random selection of opponents, or other incentives to keep things going. Restricting challenges to neighboring ranks slows down the dynamics. We will encounter this trade-off again.