Since someone brought it up, I think if it's Beau talking in her head, a voice she instantly recognizes and trusts wholeheartedly, then I think Yasha would be okay with it. Ironically, Yasha had said "I've had a lot of people in my head and I don't want to give them you."
But what if Beau's letting herself in and Yasha knows it, thus she's the exception, as is Caleb? I think she'd be okay with it if it's them. It would also be a means to balance out all the BAD people that were in her head to have two good ones, people she loves, inside to comfort her when she's in a bad place emotionally and mentally.
Also, it'd be like whispering sweet nothings in her ear only it's in her head and thus impossible to ignore. Especially if it's Beau.
Yesyesyes! I agree i think once they talk/discuss perameters of the voices itd be super healing for yash. And using it in silly ways also would be adorable. I think theres enough trust, as long as beau or caleb dont spring it on her !!