++ for the following reasons:

• Better usability for people reading the Monastery (including those without accounts, if a sensible default is chosen).
• Less tax on moderators to consider nodes for addition of <readmore> tags.
• Puts the subjective judgement of "how long is too long" on the person affected by the length: the reader. In other words, where should the poster place <readmore> tags to simultaneously cater to monks who each prefer a different length?
• Development and maintenance would likely be rather small compared to the benefit.