By no means should infants have to stay home. But to drag a baby under 6 months old around the park in the heat of summer for 10-12 hours is too much for me. I picked that answer and I stick by it.
I have a child and have friends that have FOUR children, so I'm not unsympathetic to the needs of the entire family and not just one member. However, very young babies do not need to be drug through WDW - it's hard on any adult, much less a newborn IMO. Bring a sitter with you, hire one at the resort or maybe wait a year or two. I took my DD when she was 2 (TWO!) and it was a very hard day. We're locals, and for her most times of the year it was always far too hot, far too overstimulating and just too much unless we only did 1/2 a day.
I am not judging anyone here, do what you want - it's a free country and none of my business, but when I see infants in a snuggli in July, I'm always praying there's sunscreen, lots of shade and plenty of long breaks involved.
IMO.