Lots of informative posts on this thread.
Ime the problem for many people seems to be a very binary way of looking at languages: either you are (completely totally, in exactly the same way for both languages) bilingual or you are not-bilingual-and-there-is-no-point.
Which really, really isn't how language acquisition works. It's to do with all sorts of factors, exposure, motivation, experiences in later life.
My exposure to English in childhood was limited to the studying of Ladybird books with my mother twice a week, being made to memorise vocabulary and irregular verbs. I now teach and examine English undergraduates, I coached dd in her Shakespeare monologues when auditioning for drama school, I write my own books in English, and I tweet and MN relentlessly in English. My accent is barely noticeable and most people miss it. Because I had that motivation and made the most of later opportunities.
My own dc were brought up with a mix of languages at home, exclusively English outside the home and exclusively Swedish when visiting relatives twice a year. Their Swedish accents sound native (dd has more of a regional accent than I do), but their vocabulary (and occasionally grammar) could probably be better for their ages. A few months in the country, or a job where they needed to use it, would almost certainly sort that.
I was also taught German as a child (reading German children's books and reciting the poetry of Heine). While I have never lived in Germany, so never become a confident speaker, I can read a German novel and follow the conversations of (recently acquired) German SIL and DB without difficulty. It doesn't matter that I can't present myself as a balanced bilingual in German: it still enhances my life to an enormous extent. Everything you learn enhances your life. With the right attitude nothing is wasted.