Maya does it, 3Ds max does it, XSI does it. Why ? Because it just works better than going to NLA editor and move dots around. You can skip alot of going back and forth from IPO to the NLA by having it all in the IPO Editor.

Like in Maya, you can select multiple bones and rotate them all at once. Instead of rotating each bone individually, saves
time when animating.

When animating a character, it would be VERY useful to be able to store more than ONE pose in the buffer. To be able to store multiple poses would speed up the animation process many times.

For example: A pose that involves R_should and all the L_Toes. Or Head, neck and L_arm_low.

The point is that when you animate you find poses that you want to keep ! Instead of moving then to a frame number far away, and still you have to select all the right bones AGAIN. This is where the selection manager comes in to store only the selection of bones you have in the POSE buffer.

All the above would go well in hand with the Multiple Curves in IPO Editor.

3Ds max's Character studio does this and its one of their selling features. It also shows you a little thumbnail of the poses you have in there, and you can name every pose you have in the buffer ! This would of course demand more space so that it has to be moved into maybe the buttons area.