Hi Sanjay,
That depends in part on how mission critical is the small application to the integration scenario.
The application may be small but if its critical to the scenario then a JCA adapter is still justified in my opinion.
You are right in asking the question in terms of the cost involved in developing JCA adapters and their usefulness.
I personally prefer to use an adapter framework that can be implemented as a JCA adapter when required or an
EJB, or a standalone component, depending on the integration scenario.
Also, with more resource adapter vendors the cost of adapters for ERP and other packages should go down.
Atul