Of course it can be hugely beneficial and I would have loved to have done it, but it would need to be done right as it can cause the child mastering neither language and that isn't a good solution. http://www.raising-bilingual-childre...nfo/pros-cons/ That website seems to have quite good knowledge about being bilingual without being too biased about it from the looks of it.
I grew up learning multiple languages at the same time. Parents and paternal grandparents spoke Dutch, servants spoke Sundanese and Javanese, and at school we spoke Indonesian. I started to learn English at 5th grade as part of the school's curriculum, and started taking private lessons in Cantonese in 7th grade. Due to complete lack of practice, the Sundanese and Javanese are completely gone. However, back in high school I was able to talk fluently with our servants using their own language.
At this stage of my life, English is my primary language. I can still speak and write the formal Indonesian language fluently, but colloquially I am way behind time. I can still read literatures and watch movies in Chinese and Dutch with ease, but holding a conversation would require major effort, concentration and conscious thought. Rather interesting actually, since at one point Dutch was second nature to me.
I married a Midwest wheat blonde girl and had two children. While they were growing up we gave them every opportunity to learn a second language. However, neither showed any interest in learning another language. Oh well. We tried.