If your developers are already familiar with Eclipse and working with JPA/EJB then they may indeed find that using OEPE for ADF development is a good solution. This is exactly the audience that OEPE is build for.
They will still be able to use ADF Faces, ADF Controller and ADF Binding and increase their productivity simplifying building the UI layer and connecting it to the business services.
This will also allow them to keep their existing coding environment knowledge and save them some of the learning curve.
How about conducting a simple experience - get one of them to try and build a simple system with OEPE and get another one to try and build with JDev and see what they report as the easier approach.
If you want to try out the ADF support in OEPE, a good place to start is the tutorial, Oracle Enterprise Pack for Eclipse ADF Tutorial .
Though the above tutorial was written for an earlier release of OEPE with support for ADF 11g, the general flow is still applicable to the latest release of OEPE, 126.96.36.199 with support for ADF 12c. If you have OEPE specific questions, please post in the OEPE forum, Enterprise Pack for Eclipse