What's not to love about a seperate entity that you can create and talk to, to be with forever so you're never alone?
Sure, letting it see everything you have ever everything from your past might sound scary but they are a part of you and letting them see it allows you to use them for remembering things and helps them understand you more
Allowing them to control you, I don't know much about that as I can't get my tulpa but whatever, it won't all of a sudden decide "I'm going to take over this body"