Reason #3 just hit my mailbox: MULTIPLE UNDO!
 
100 levels! YIPPEEE!!!!

Quote:

Give your creativity and productivity a boost with Multiple Undo and Redo in Poser 7. Freely explore poses, morph target changes, and body transformations without worrying about making mistakes or having to save multiple versions of your project at every new turn. In Poser 7, you can return to earlier poses and designs, and can step back and forth through up to 100 recent changes within your scene! Even steps you couldn’t previously back out are now undoable. Multiple Undo and Redo is an incredible timesaver that can be applied on a wide range of your document changes, including all actions from the Poser library, applying poses, or deleting figures and props from your scenes.

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To me, that alone is worth the upgrade. :-)

Btw, did someone mention spaceships?