Having spent a portion of my childhood in Europe I saw first hand how much of a difference their default state of speaking 2 or 3 languages has on a society. Trust me, learning a second language is worth the time.
Where are you from in the US? Im from the Atlanta area and bilingual spanish/english. Most people I know from georgia, florida, even alabama can at least understand spanish just from being around it so much. I really think to not know spanish you would have to actively try to avoid it - especially if you are young and from the city. In atlanta you can easily live without knowing any english, when friends came from south america we always found people fluent in spanish most native (at Starbucks, McDonalds, Grocery Store, shopping, etc).
Don't forget that part of the reason that English is well known across the globe is because the US is a media powerhouse. The web is practically english, US blockbusters are seen worldwide, and it's essentially the language of international business.
Because you want to sell to people who speak a language other than English?
Because you want to buy from people who speak a language other than English?