Ecumenism in Christianity aims at unity between branches of Christianity. It’s important to distinguish, however, between unity in spirit and unity in organization.
That Christians have better dialogue, exchange of ideas, and a sense of kinship all sounds like the right approach to me. However, there is also a sense in Christianity that there needs to be unity of organization – one church, say. This to me seems an error.
First, understanding various denominations is important for 3 reasons. 1. It allows you to see what’s working in another approach, and consider how to modify it and apply it yourself. 2. It allows you to see what’s not working in another approach, avoid it, and also give constructive feedback. 3. It allows you to cooperate or coordinate on areas where there is overlap between your approaches.
Having said that, diversity of approaches can be highly useful. The basic idea is the same as in free markets. Different approaches = experimentation = success of certain approaches, iterated. This is similar to various ideas in evolutionary biology – having a certain amount of diversity of approaches is useful.
Therefore, it seems wrong-headed to me for Christians to want all Christians to move to the same church, or even to see it as primarily a competition between churches. Christianity, as a whole, probably benefits from different approaches, learning from other approaches, and being critical of approaches that seem obviously wrong. Understanding what is actually happening in other churches benefits all these points, and ecumenism properly should be about better understanding of other approaches with an approach to unity in spirit.