I have to respectfully disagree that not being a parent means you know NOTHING about parenting. I have no carried a child to term but I have been a nanny for 20 years, have a degree in ECE and extensive training, have taught parenting classes, have at various times taken custody of THREE different children of friends, am a GAL (guardian ad litem), care for at least 2 babies daily (who wear CD's in MY house). I may not have given birth (therefore not a parent) but I know a LOT more than many of the parents I know. Many of them regularly ask ME for advice. Fortunately they don't think I'm totally ignorant just because I have not given birth.
Parenting is way different.
I'll grant you that you're definitely more used to the mechanics of things. However, when you are actually a parent, it totally changes things. And I really wish that I had known that while looking down a bunch of other parents all those years--as well as getting frustrated when my advice didn't work as well for them.
Kids behave differently around their parents. You don't have a light at the end of the tunnel when you're a parent.
So yes, I agree that people who are not parents can and do give good technical advice...but I think that they cannot honestly feel the kind of empathy of parenthood that someone who has felt the rage has.
Also, you don't need to have actually given birth to be a parent, FWIW. For whatever mystical reason, I have found that most of the adoptive parents I know feel that same rush as bio kids do.
It's always different by necessity and even somewhat subconsciously (IMO) when you know they're not really yours, than it is when they are YOURS. And the kids know it too, and therefore tend to act differently.
Waaaay off on a tangent here though.