Yay - interesting debate!
I'm a translator/interpreter, and dp and I are bringing our dd up bilingual. I speak to her in my native language, English, and dp speaks to her in his, Spanish. We live in Spain, so everything outside the home is also in Spanish. We spend a lot of time with English-speaking friends and see a lot of my family. So it's pretty balanced so far, although dd is only 17 months old, so I don't know what her English and Spanish will be like in a few years' time.
I was very interested in the Translation Studies and Linguistics side of things while doing my postgrad, and actually did my dissertation on Semiotics - it is nutty, isn't it Grendelsmum! I did laugh at your post, you poor linguistic/cultural identity guinea pigs!
First of all, I think having a native language is extremely important, and it is true that there are cases of people who express themselves extremely well in two or more languages but have no true native language. I also think it's virtually impossible to have two completely native languages.
I don't think it's unnatural for a mother to speak to her dc in a language that is not her mother tongue, and it can be done well imo, provided the decision to do so is an educated one. If the mother has near native competence (and if she describes her level using this term, as MIFLAW as done, she obviously knows what she's doing) and her dc have plenty of exposure to the other language, ie from their dad, school, living in a country where the other language is spoken, then this set-up can work very well. MIFLAW's dc, for example, will have no problem becoming native speakers of English, even if she, their mother, never speaks to them in English, because the rest of their "world" is in English (I presume..?)
However, the situation that the OP describes is a different matter. I don't think someone with A level competence in a language is a good enough linguistic role model for their children to learn from. But I seriously doubt she speaks to them in French all the time OP, I think she's exaggerating and just speaks French to them sometimes. Maybe she just wants to impress you and just chatters away to them in French when the notion takes her. And even if she is kidding herself that she's going to speak to them exclusively in French, she'll soon get tired and give up, because it's pretty much an impossible task if you're not up to scratch. And if she does carry on, then her French is better than you think it is .
Lastly, imo YABU OP to think that generally it's pretentious to speak to your children in a language that is not your own. That is a typical British sentiment of tutting/tittering at people who get ideas above their station and who do they think they are anyway?? Voltaire?