I personally wouldn't say that the summary rule came out of the blue. It has been an issue for a long time around here, and I believe even a matter of debate in the past. I know that seeing a really long summary on the most recent page will actually deter me from clicking into a story, because if I don't have the patience to get through the summary, then I feel there is no way I'll have the patience to get through the story itself.
But I mean, a rule is a rule. It says right there how long the summary can be and that it'll be edited if the rule hasn't been followed. So, if someone reads the rules, and keeps their summaries an appropriate length, there shouldn't be an issue. Julilly also said here that they will be giving until April 20th for that rule to be enforced. That's a lot of time for people to go and edit the summaries themselves before it's done for them, so they are giving the authors the chance to edit their own summaries if they don't already fall within the guidelines.