The goal of the C deck - not counting the 4 characters - is to increase replayability with more varied cards in decks, also adding extra copies for much needed cards for bigger parties. It depends on personal taste, if you want it or not.
There are a few very good boons in the deck, and Lini will be especially thankfull for the animal allies. I wonder, if you purchase her without the C deck, does she still have the animal allies in her starting deck ?