It would be very useful for addons extending Controller methods without rewriting them.
The most common problem in such extending is to change some params inside a controller without extra queries and rewriting whole conroller.
For example, we want to change number of messages shown in the current page in some special forum.
If we do it before parent::actionIndex() we don't know the param 'forum' and have to do extra query to know it. And we can't do it after parent::actionIndex() because the posts are already fetched.