My mom's parents spoke German at home - amongst the *adults* - and never taught any of the kids more than a handful of household words. Some of it was wanting their kids to 'fit in' - my grandparents were both children of immigrants, and probably struggled with not having English as their mother tongue, growing up, and thought they were doing their kids a favour.

But from my mom's perspective - her parents spoke German when they didn't want the kids to understand what they were saying :D

It did make my mom VERY interested in signing us up for French Immersion when it was offered - she wanted us to be able to speak more than one language, even if she couldn't!

As it turned out, Spanish would have been more useful in my day to day life, now - but I do find knowing French helps me puzzle out some Spanish, and probably gave me a leg up in learning the smidgin of Spanish that I do have.