Is it really possible to use the transaction strategy as default? It has very limited use cases, right?

That's up to the project I guess.
This just allows an easier way to set the default than having to change hundreds of test case classes

It has very limited use cases, right?

As the docs say

By using TransactionStrategy each test method will be run inside a transaction that is rolled back at the end of the test. This can yield improved performance but requires your tests not heavily rely on static fixture data, as auto-increment values are not reset before each test.

What other limitations are you referring to?

@markstory That would be super helpful to quickly change the strategy per app without having to change hundreds or even more files to different base class in between.

When would you use this? Do you have a plugin or application that can run all of its tests with TransactionStrategy?

Yeah, any application itself as well.
Any application that already has 100+ tests would be nightmare to have to add a middleman test class that would extend and modify this protected method.
Having a global config here switches it out without the need for massive refactoring.

The default one as given right now is sure not helpful beyond a few tests, as it becomes a pain to work with, everywhere relicts after test runs breaking the next tests.

@markstory can we ship this with tonights release maybe?

Docs update in cakephp/docs#8025