I spoke no Spanish for the first two years I was here. Now I am by no means fluent but conversational and it certainly helps to broaden the experience of living here.
It seems to me many on the forum are used to living and spending time in five central neighborhoods. I live in Versalles and spend most of my time in Versalles, Villa Real, Devoto, Villa del Parque, Liniers, Moreno, and Caseros. I can assure you, people in Liniers and Moreno have zero interest in practicing their English with you. If you don't speak Spanish, at best you'd be a curiosity and at worst you could be putting yourself in danger in those barrios by walking around talking in English.
Even in Devoto not getting a curious glance when I talk English is rare. The first two years I lived in Villa Raffo I was basically on my own. CABA has over 3 million people and the greater city around 13 million, head outside those central barrios and you'll actually find the city is not truly bi-lingual. Or perhaps more accurately, if it is the people don't want to talk to you in English (why would they, by the way).
That's not to disagree with Ceviche saying Buenos Aires is the most bilingual city in South America. Just think about it though, that "fact" is meaningless without any data. I am by far the best French speaker in my house, I know 10 words.