Sure, if a screen shot removes ambiguity or clarifies an answer to be correct. If the criteria of a question hinges on the usability of software A not being ideal, with hopes that a yet unknown software B that accomplishes the same thing with a better UI can be found, then screen shots are probably the best way to illustrate that.
They're also considerate, I'd love a screen shot to help in my mental selection process of the things I'd like to try.
Don't let a picture replace a thousand words. Images can't be searched for context or meaning, so we need lots of great text explaining that which you're showing in order for content to be easily discovered through search, which is likely to be the bread and butter of this site after private beta.
A screen shot would make your example a bit more useful, but it doesn't fit the lacking real testimonial for the software we hope to see in answers. That answer needs more experience in it, and a picture isn't going to fix the lack of it.
Don't go as far as demanding them if the answer otherwise gives all needed detail, and on the flip side of that, don't be shy about adding them where and when you can when it makes sense.