Anecdotes are just that. I know plenty of people that can dominate two languages fluently.
Recently read a study where a stroke patient can recover faster if multilingual or that dementia can be kept at bay longer.
1. Portocarrero et al. (2007). http://www.sciencedirect.com/science/article/pii/S0887617707...
2. Gatt et al. (2015). http://ijb.sagepub.com/content/early/2015/03/13/136700691557...
how do you think i knew they had poor grammar in both languages?
most people can't even speak or write a single language well, of course a huge number of people of average intelligence/aptitude who are forced to speak two languages are going to be terrible at both.
how many people can 'dominate' their native tongue? not many.