The answer is, of course, ‘it depends’ – as (nearly) always.
First of all, what is meant by the term ‘formal test cases’? I’d argue that typical unit tests (i.e., those written using Minitest in Ruby, JUnit, and friends) are formal tests (the fact that they’re written in a programming language qualifies them as ‘formal’).
Then there may be test cases that are executed manually but still may be called formal…
Either way, it depends somewhat on what product is tested. For medical hardware or software, I’d definitely want some formal aspects to be respected (to say the least).