Here's some first-hand experience of being raised bilingual, for what it's worth: I grew up in an English-speaking country, with my mother speaking Russian to me, and my father speaking English. When all three of us were together, we all spoke English, as my father's Russian isn't very strong. The result is that I can speak and understand Russian fluently, and read and write it at a push (but this takes a huge amount of effort on my part.) I wonder if my written Russian would have been better had we only used that language at home. Positive side-effect: I became fascinated by language, ended up with a degree in an area of linguistics, and now have a job where I get to make creative use of language. Negative side-effect: I found learning a third language at school tougher than I should have, because my brain would often switch into my 'default other language', Russian. This resulted in some awkward moments during spoken exams, where I would respond in Russian when questioned in French.