For me is the criteria the quality of the informations which is added.
- It it makes the intention or requirements better to understand, it is normally a good thing.
- But it isn't acceptable if it invalidates an given answer.
I have seen today some questions, where the OP hasn't done the effort to write clearly his requirements and has changed the goal, after he received some answers.
This bad habit shouldn't be supported.
Answering a question takes time. This should not become invalid only because the OP hasn't done his homework.
To obtain good and valid content it is not acceptable that the answer contributers are getting discriminated, because the questioner hasn't taken himself the time to express all his requirements.