Currently, drupal core’s menu blocks just print an entire tree of the specified menu. If you want just a part of the that tree, you can't do that with menu.module's blocks. Its all or nothing.
But look at drupal's themes. They have special variables that can show the primary or secondary levels of a menu. (The secondary level of a menu is just the children of the active primary menu item.) Doesn't it seem odd that we can't reproduce even this simple use case with menu.module's blocks?
Let's look at another common scenario: graphic designs for websites often have the primary links as a list of links along the top and then have the 2+ levels of the menu in a sidebar. Again, not possible with core menu.module blocks.
Since the menu.module holds all the navigation links, Drupal core should enable site admins to expose these links in any way that their site architecture requires. Right now, we are only enabling a single navigation style on sites. Talk about “that site looks like a Drupal site”!
Also, menu.module pollutes the admin/build/blocks admin page by adding a block for each menu in the system. Even if you don't use any of those blocks. I've had sites with 7-8 menus, and all those unnecessary blocks on the blocks admin page hurts the javascript performance. :-p
Fortunately, the Menu Block module in contrib has all the necessary configuration to allow flexible menu-based navigation as blocks. It also only adds a block to the block admin page when you actually want a menu-based block and not before.
So, we should move Menu Block module's functionality to core.