Nonsense! Knowing the grammar is fundamental to successfully speaking a language. How you learn it is  not important, but without it your level will be poor. 

I disagree because I have observed students become fluent when they learned English grammar. Grammar provided the framework.  All they needed then was to add vocabulary. 

I completely agree with you. One of the reasons why I failed to get far in the languages I was learning at school was because of grammar. The teachers shoved it down our throats from the very beginning.  You can't expect me to write an essay in German if I don't have enough <em>passive</em> vocabulary to do so. 

@arni:
Sometimes I think a little before I say something. More often I reflect on what I've already said and realize what I said wrong. But I wouldn’t beat myself up because of my mistakes. Instead, I try to focus on how to say it right the next time. The trick to speaking is to make it fun. Give ourselves some room for mistakes and freedom to be silly. Focus on getting our ideas across (not so much on the grammar). It is like swimming; we don’t consciously think about how our legs should kick water or how to change breath; we just do it and make adjustments along the way. Get used to making funny mistakes the way we get used to the taste of pool water. If we are too afraid of choking on water, we won’t practice enough to be great swimmers. Mistakes don’t bite and no one is there to grade us. Relax and enjoy your conversation with friends/teachers on italki. You’ll be just fine.

If you want to speak English just like an ordinary/common american you must interact with them always if not possible you can interact with them through online or just watch tons of english series and movies. You must speak the way they do, use the terms and expression that they are using. However if you want your english to sounds like a professional specially when your aim for learning English is for work and business you must study grammar. If you don't study grammar just because your aim is to become fluent you might sounds cool but you don't sound smart. Grammar, vocabulary and practicing should always come togther to be fluent.