XML is the single fundamental technology in Web services on which everything else is predicated. A good working knowledge of it will help you in the long run where tools and toolkits fall short, you will be able to jump into the breach.
Everything in the Web services architecture is governed by schemas. Every self-respecting architect and developer should understand XML Schemas, at least to the level presented here.
XML Schemas are best used to describe type systems first and document layout second.
When using XML within your own applications, make it a natural part of development to write schemas to accompany the documents. A good rule of thumb is: A document is useless without its schema.
XML processing technologies are a commodity don't reinvent the wheel unless you specifically cannot achieve your goals with off-the-shelf components.