Glad you agree. I think the following would be great for any node, with three radio button choices that the user must select from:

( ) Set teaser to <input some number> characters. (If left blank, Drupal will use the "Length of trimmed posts:" setting, under Post settings.)

( ) Set teaser to end at "<!--break-->" tag. You must add the tag yourself. <Add warning about what input formats are needed to work well with such tags.>

( ) Input custom teaser. <Follow with input field>

What could be better? : )

Now, do you know anything about the process of actually pushing for some change to be made in core? I would assume there's some formal way of submitting such a request, other than making wishes on a small web site.