As a themer, I spend ALOT of my time in the node.tpl.php (or node-custom.tpl.php) file doing layout. Often times, however, I'm not doing one layout in that file...but two: One for the full node, and one for the
theme teaser (using if($page), etc...)
It would really clean up the node.tpl.php code alot if we could have a separate template for the teaser recognized out of the box. The idea would be that each node template could also be accompanied by a -teaser.tpl.php template where all teaser layout could be done. Of course, for those themers who wanted to continue doing both in the node.tpl.php they could keep doing that. But, by having this separate -teaser.tpl.php available, I know that it would clean up alot of theme code AND would be a way to do teasers that many more new themers would understand (right now, many folks don't even consider doing an if($teaser) and therefore don't know how to create a separate teaser layout).
So, to clarify: With this addition, we'd be able to use a node-teaser.tpl.php or a node-custom-teaser.tpl.php as a way to layout our teasers in their own template.