Nice when you can do so, but try to find, for example, a plot summary of an obscure Victorian play, or, for that matter, in many cases a plot summary that includes details of the ending. Also, there's numerous cases where I've seen inaccuracies (or, at best, descriptions that reflect changes made in a particular performance) in summaries of a play; if we're not using the original work, and don't have sources to contradict it, are we forced to include inaccuracies?
If it can be done, it should be done, but it really, really isn't as big of a deal.