That's a good question. I am bilingual, French and English. My parents speak French to me so before starting school I grew up speaking in french always, now I speak English permanently. However I can speak it fluently and understand perfectly. Like other people said having more than one language at hand is very useful especially if the parent speaks anything other than English, no harm in teaching the child. My cousin's mum is French Caribbean, dad Pakistani, so they both speak their own language to their children and the eldest can speak Arabic, Urdu, English and French! I think it would be worth teaching my kids French too as I have a lot of French speaking family. Another example is I have a 12 year old cousin who has a French speaking dad. She can only just understand a few words in French but can only speak English. It's tough for her to understand and talk to members of the family like our grandma back home who can only speak in French so communication is VERY limited sadly. Take this as a lesson guys!