Tests are also code, thus they also add maintenance costs. IMHO, usually about 8-15% of code (depends on the project) may need automatic testing. If the code can be quickly tested manually, maybe you should think twice before writing tests. The code that changes too quickly (because of requirements changes as mentioned in the article) is also not the best candidate to be tested automatically since what's the point of regression testing if it's almost rewritten from scratch. A better candidate that is worth to be tested in automatic way is a code that is critical (medical equipment, bank systems, airplanes, etc.) and changes slowly/gradually enough (so regression testing will make sense here).