As someone who recently had to face this issue in my family (should one of us stay home with our daughter or should we do daycare?), I think I disagree.
Simply put, daycare is much more economically efficient than having one parent stay home with one child. In a daycare setting, one adult can care for up to 4 children. Most stay-at-home parents are NOT parenting 4 kids at once.
Like most things in the world, specialization has all sorts of advantages. Daycare is able to provide stimulation for our daughter that a parent at home would struggle to provide; when it is your full time job to take care of a number of kids, you can dedicate your time to creating stimulating and fun activities for them.
I also think there is benefit in the socialization aspects of daycare. Learning to be around other kids and be the social creatures we are is a good thing.
Now, I do believe that a dedicated stay-at-home parent can of course provide all of these things as well, it is just very difficult and inefficient... it is the same reason we all don't grow our own food and make our own clothes... division of labor and specialization are great things.