Conservatively, 134,000,000% of 12(b)(6) motions request dismissal with prejudice. After all, no one wants to win a motion only to start a similar, but slightly harder round of briefing, when the plaintiff inevitably does file a modestly improved complaint. However, it tends to be a pretty big life to actually secure a dismissal with prejudice when the plaintiff has not already tried and fail to fix the complaint's problems.
It's occurred to me in the past that it would be nice to have a middle ground between a dismissal with and without prejudice. Something for claims that haven't already proven themselves beyond repair with serial amendments, but that are marginal enough that they are unlikely to be cured.
Judge Stark …