You can refer the document " SAP Guidelines for Best-Built Applications that Integrate with Business Suite ".
In Page 84, 85 SAP clearly recommends usage of Web Services / Enterprise Services in BPM process design using NetWeaver BPM.
I will leave you to guess if WD Java / Java based apps are better prepared to consume enterprise services and web services.
Also, if WDA is used as the UI in the BPM process then solution will be on different stacks / may be server. Reason: BPM runs on Java stack so its better to have the UI on the same stack.
Regards,
Sharath