As someone who loves art in general and wants to learn more and more about any kind of topic, she would be the perfect teacher for me, and she would be happy because rather than a master she is looking for a pupil.
Also I’m sure she would love to be summoned in Italy, I don’t live in Vinci but she would still enjoy her new life, I’m sure of that.
And I guess she would be useful in a Grail War, she’s not the strongest caster that exists but from what I remember her NP, lore-wise, is very strong, I would feel save having a genius by my side
If not Da Vinci then Mori I just love the guy, I love his personality and how much loyal is to his master. I’m not sure if having a crazy berseker as a companion would be a nice idea, but for Mori I’d take the risk