[PeterisP]

Unit testing makes sense when there are some units for which you can define correct behavior. E.g. you may not know all the requirements yet but if you can isolate some components that you're going to definitely need, then you can write unit tests for them. While you're still iterating about what that unit is going to do (and whether you'll have that unit at all) unit tests make little sense.

However, in quite a lot of common scenarios, e.g. a simple user-facing CRUD webapp, none of the custom content functionality is expected to be stable enough, it's all still in flux (and thus any unit tests would have to change at every iteration); and all the components where from the beginning it's clear how they should work (e.g. user authentication, sending of email notifications, etc) aren't really custom, so they're already written and tested, you're going to simply use them from your framework or standard libraries.