What a fascinating discussion and perhaps I can offer a fathers perspective.
I'm English only speaking, my wife is fluent in English and Welsh and we live in London, England. Now, we both speak English to my 5 month old child when we're at home together and my wife speaks Welsh when they're along. So I guess you could call this the 'closet' language.
I'm happy for my wife to teach our child his Welsh heritage but I have my reservations. I'm very much a hands-on dad and want us to both be working in a tight parenting partnership and I feel speaking English together is working well.
However, the main problem I find is that when we visit my wife's family in Wales who are multilingual in Welsh and English, they will all speak Welsh and I'll be unable to follow any conversation. Now as a parent, not being involved in any discussions or decision making is very difficult. I'll often be sat there staring around the room and all of a sudden I'll be told we're going out for a walk with the little one. Where?, for how long?, who with? - I don't know. And being a parent it's very difficult feeling out of control.
One option is to learn the very difficult language - which is a very tough task considering the lack of time I seem to have since become a parent.
I wonder if anyone can understand this point of view or offer any advice.