Because the rainbow parenthesis alternate in frequency of brightness, its actually really easy to distinguish which is which, unless you're severely colour blind.

I can't tell some apart, but because they've got a different colour in-between, it makes it easier to jump between start and end of expressions. Being able to box blocks in my head faster.

That being said, I always have to tweak accessibility settings anyway. Change of font, change of size. Having to toggle off rainbow as well doesn't seem to really add to the large list of things.