Grit, Gravel, and Oyster Shell.........................…

I read up on all the birds I bought prior to purchase. I own 4 parakeets (one ped yesterday,husband left his window open and I think he froze to death while I was away), a love bird that is under 3 months old, 2 paroletts, 2 sun conures, and a atoo. I read up on each bird on the internet through numerous sites as well as bought books to go along with each so I knew better how to care and what to expect from each of the little guys. However I read here on yanswers many ppl that claim to be "vets" saying that pet birds do NOT need gravel-grit- or oyster shell in their diets. Everything I read says they do need it that it aids in digestion and in reducing hyperactivity. What is right all the books or the "vets" on here I give all my birds gravel mixed with oyster shell and have been for yrs without problems. Please clear this up for me thanks...