Following up from IRC...
regarding the $force parameter:
drupal_render() is smart in that it tracks when it's already output something, and won't output it again. This is very useful, because in form theme functions, you can control the order that fields are rendered, but also just print out "the rest" at the bottom to get tokens, submit buttons, and whatnot.
The $force parameter would indicate, "No, I don't care if I already told you to render this, do it again anyway." So this would allow someone to print the same node property twice in their node.tpl.php file.
We discussed on IRC and are pretty much agreed that this would be an extremely rarely-used feature (how many times do you render the comments of a node twice?) and the API would be simpler without it. People who need this sort of functionality can always store the result of a drupal_render into a variable and print it elsewhere in the page.