The thing, though, is that "nativity" can get fuzzy depending on a variety of circumstances. I think a lot of new bilingual parents assume that all bilingual parents are like them: strong speakers of a native tongue who had kids shortly after a recent move to a new country. That's not always the case: there are bilingual parents that moved to the new country as children and may have varying degrees of mother tongue attrition, there are second/third generation parents and there are multilingual parents (e.g. immigrant parents whose parents speak a dialectic variation of the official mother tongue, or even completely different languages altogether)
For many parents, the primary goal for promoting bilingualism is purely practical: speaking w/ grandparents and other family. For the most part, the child will still need formal complementary studies later in life if they want to truly master the parents' mother tongue to a grown adult's level of proficiency, especially for languages with little or no overlap w/ english.
IMHO, the benefits of nurturing a mother tongue are largely coincidental: e.g. romance language learners get an advantage over pure english speakers from understanding latin-based radicals (especially for stuff like scientific nomenclature). But for many languages, maintenance comes with an opportunity cost aspect: my wife speaks Mandarin and actively maintains it, but that comes at a cost that her English vocabulary is not as well developed/polished as mine (as a romance language speaker who doesn't need to do virtually any upkeep wrt writing/reading skills and who enjoys vocabulary overlap between english and latin-originated words, etc)