(Idle curiosity... Are you the same Weirdo from the ADB/FedCom Forum?)
You should buy it because it is 99.44% perfect, which is more than 98%.
...
Oh, you want more detail?
Unity builds more upon Admiralty than upon Nova, it is true, but there are some Nova refinements involved. It's not strictly an update to Admiralty. That being said, I suppose the rules modifications COULD have been presented in Rules Annex II, but for a couple things:
1) That still would have required you to buy another book.
2) Presenting it as "(Admiralty) Rules Annex II" would have implicitly maintained the Admiralty/Nova split, which was the primary problem this edition is meant to address in the first place. It is called UNITY, after all.
What can't you live without?
A) A finely-crafted rules set taking what was best from each prior iteration of the game and building something even better. A comprehensive sequence of play is provided, and the rules are fully cross-referenced.
B) Our most streamlined and balanced construction/point-costing system to date.
C) Dedicated seeking weapons.
D) Six brand-new scenario templates, adding to the six from previous editions.
E) 48 sample ship displays from 14 different fleets across five separate universes.
If that isn't worth $13, I don't know what else to tell you. 