On the contrary, I think custom tags are an excellent idea. They can be used to encapsulate a load of design-time instructions and when executed, can collect metadata about their attributes, and create complex markup and script.
I think a fully integrated Java/Ext framework is an excellent idea.
I'd like it to also integrate Hibernate for easy access to a database.
With Hibernate, it would be a doddle to create a tag which could create a Grid from tags and attributes that are an analogue of Hibernate capabilities like the Hibernate "Criteria", "Restrictions" and "Projections" classes:
Which would result in
<ext4j:column property="name" header="Name" width="100"/>
<ext4j:column property="language.name" header="Language" width="100"/>
<ext4j:restriction type="LIKE" property="area.code" value="NG%"/>
<ext4j:order property="name" direction="ASC"/>
And "results" would be a java.util.List of Object where each Object in the array was the column value.
List results = session.createCriteria(Customer.class)
.add( Restrictions.like("area.code", "NG%") )
.addOrder( Order.asc("name") )
That could be exported via DWR, or Json or XML.