A lot of people "cheat" and develop their schema without developing a model:

• A schema is better than no schema.

• A carefully thought-out and document schema is better than just a schema.

• A schema that is designed against a model is best.

But "cheating" is OK in some contexts:

• Application specific documents.

• Ad-hoc documents.

• Simple narratives or interchange documents.

• Situations where you need a schema for typing/validation.

But there are many application domains and standardization efforts where you are better off developing a model first.