A post should, ideally, be completely comprehensible from only the text. Text
can be searched, indexed, copied, zoomed, easily edited, and is accessible to
You can't really do any of that with screenshots.
There are certainly situations where a screenshot could help to further
clarify the problem. Some examples might be
or this question.
In all of these cases, the text is enough, but the screenshot might clarify the problem.
So should their use be encouraged? We should encourage clear questions that are easy to understand. When will this require a screenshot? It's hard to give a list of requirements. Use your own judgement ;-)