Isn't the idea with unit testing forgotten here? The point is to validate the blocks you build and use to build the program. In order to make sure you've done each block right you test them, manually or automated... Automated testing just is generally soooo much easier. If you work like that, and do not add test after y,ou've written large chunks of code you should have constructed your program so that there's no overhead in the test. Advanced test which does lots of setup and advanced calculations generally ain't the test fault, but the code itself that requires that complexity to be tested.

Wanna underline here that system tests are slow, unit tests are fast.

This said, i agre that you should throw away tests in a similar fashion as you do code. When it does not make sense don't be afraid to throw it, but have enough left to define the function of the code, in a documenting way. Let the code /tests speak! :D