Shadow had the same feeling that Yugi had about Jaden. Completely friendly, optimistic, and very passionate. In some ways, Yuki may be similar to Joey, in a way. Plus, she was fascinated with Yubel. So, why not allow these two a spot in their journey. Perhaps they could help them into saving Solomon Muto. Besides, a friend of Yugi Muto, is a friend of hers. She also wondered what kind of duelist he might be. 
How does he play it? What's his style? 

With Yugi asking for her consideration, the genie bowed her head, and closed her eyes passively, to show her acceptance of letting Jaden join them. 
[i]"We'll need all the help we can get"[/i], she simply to the two duelists, and the she-demon. Like Yuki said, "the more the merrier".