In short it was easy to misuse global v8::Persistent<T>`handles leading to memory corruptions and such. Unfortunately making them safe to use also means making them hard to use directly. Previously they were just interchangeable with normal v8::Local<T> handles for things like calls.

https://groups.google.com/d/msg/v8-users/6kSAbnUb-rQ/e-GI0b1...