If your kid is very bright, it's better for him/her to be among general peers and experience what it's like to be top of the class. In my opinion the confidence that this instills is more important than "not being bored". All the serious learning happens in high school anyway.
Some parents who rabidly pursue gifted programs deep down know that their kid is not special, but are hoping that the giftedness of other kids in class will rub off on their kid, or that the higher level of education will push their kid from average to above-average. That's also where the "smart kids are bored in class (and hence not doing great)" comes from.
Nah, pretty much everyone of slightly above average intelligence spends most of school bored.
Some non-exhaustive examples:
1. Bored because they already knows the material.
2. Bored because the particular in-class activities are not engaging but they'll make more progress at home.
3. Bored because the entire subject seems like pointless memorization.
4. Bored because of a neurochemical issue.
I went to a magnet high school. The very smart kids have a lot of fun running circles around everybody else.