Deleting all posts is a fairly resource intensive action, particularly if the member has made a lot of posts. For each post you delete, you may need to rebuild the topic it was in (in case that post was the first or last post in the topic), and then you need to rebuild all forum data for all forums they posted in (so that the number of topics and posts in each is accurate), and then you need to rebuild the forum stats (so the total number of posts is accurate). When you delete all members' posts from the ACP, it does it in cycles of 50 posts, and redirects to a new page automatically where it does the next 50. You'd have to program something into the warn panel to be able to replicate this, as deleting all posts from someone who made 5,000 posts on the forums could not be done all in one shot.
I would say that for the frequency of how often most folks need to do this, and this is just going off my experience here, it's enough that an Admin can do this option in the ACP as it comes up. Whenever I have needed to delete all posts from one member, I wasn't doing it when I was just warning them, I was doing it when I was banning them via the ACP, where I would have the ability to delete all of their posts right there anyway. Just my 2 cents.