Is there any reason to buy the Ascension base game?
Not to be Captain Obvious, but you would buy CotG if you want the cards.

I quickly glanced at the card list and a few "must-haves" stuck out:

Enlightened: Master Dhartha, Twofold Askara, and Tablet of Time's Dawn (though you might NOT want the possibility of infinitely many turns...)

Mechana: all the constructs in this set are valuable, Burrower Mark II maybe not as much.