You're right, I don't think a kid would be "corrupted" by hearing a parent speak a language that is not the parent's native tongue.
It's just that, how much time does one actually speak to their kid during a day? It's not like most parents take the time to sit their kids down for an hour of "English conversation" practice. Maybe people do that over dinner, but otherwise how much does a parent really talk to their kid aside from saying things like "dinner's ready", "did you do your homework?", "go brush your teeth", etc. If you add up everything that you say to a young kid over a day, how much English are they actually being exposed to? Again, unless you're the type of family who thoroughly converses over a long meal, you probably don't say a whole lot to your child over the course of a day.
If they grow up in Argentina they'll be hearing Spanish all day - at school, with friends, on the street, & on TV. So why not just make a habit of only speaking English at home, so that they can be as exposed to it as possible? It really comes down to that - making it routine so that one doesn't even have to think about it.
We are creatures of habit & once a habit has been formed (like allowing your child to speak his/her native tongue at home instead of your native tongue), it's extremely difficult & frustrating to reverse.
One of my cousins in the States has made many attempts at teaching her kids Spanish by speaking it at home. But she's so used to speaking to her kids in English, that she just forgets to speak Spanish - despite the numerous post-its she has around the house that say "Speak Spanish!" lol. She just moved to Chile for 6 months in hopes that her kids will pick up the language, but no matter how much her kids learn while they are there, I can almost guarantee that unless they make a conscious daily effort, they'll probably go right back to speaking only English at home. Why wouldn't they? They always spoke English in that house.
Plus most kids (at least in the States) don't see the benefit of knowing another language. And seeing as they lack the motivation to learn/practice, the parent should really not make it optional for them.
I knew a lot of kids growing up who spoke Chinese, Japanese, or Farsi perfectly (along with English) because their families had strict rules that the parents' native tongue would be the only language spoken at home. Like I said before, I wish my parents had also instilled that rule because my Spanish is nowhere near perfect. And oh wouldn't it be nice to be practicing my French instead of continuing to work out the kinks I have with Spanish!