Should developers write unit based tests cases prior to developing the test

Background

We are currently moving API testing from the Technical testing team to the development team. The technical testing team has a formal process where they write the test objectives and test cases prior to developing the script in SOAPUI. Therefore analyzing the specification and supporting requirements document to derive the tests.

In the new world the developers will code/point and click their unit tests and there will be no other QA other than the UAT of the consumption of API, which can happen between a month to three months after the API has been created. The API is not an open API, only for third party suppliers and internal web and app resources.

There is an argument that this is not needed if the developers are writing the unit tests, so do not need to have an analysis phase or write test cases prior to setting them up in SOAPUI or coding the test.

We create enterprise level financial websites and applications. The teams is 30+ strong.

Question

As developers and perhaps the odd QA person on the site, what is your opinion? What are your experiences of Unit testing and writing tests cases?

Regards,

Tim