Maven just brings in dependencies, not integration. So if you want something outside what servlet provides you'd likely want to use WildFly Full.
For example say you want JPA, CDI and JAX-RS. You'd have to provide implementations for each API. Those implementations would have to know how to integrate with each other. In JPA if you want to use a DataSource you'd have to bring JCA and Transactions. Those too would have to be integrated to ensure the transactions are managed across all container components.
If all you need is just a servlet container, then you probably don't need WildFlyFull. Once you step outside those needs, say just a CDI, then you more than just a servlet container.
James R. Perkins