I think the most important principle here is choice. People should ideally have the opportunity either to work, or to stay at home with the kids. And I don't think they should be judged for making certain decisions.
Having said that, I do believe that a woman who opts to be a stay-at-home Mum permanently is selling herself short. Sure, when the kids are younger its great to have at least one of the parents around full-time. But when they're a little older, I think it's good for both Mum and the kids for Mum to have a profession.
My Mum did not work until I started school, she went back into the workforce and is a senior civil servant in the Treasury of my home state (back home in Aus). As well as being my Mum, whom I can go to for love and support, I also have tremendous respect for her as a professional, I know I can always ask for her advice on work issues, on feedback on some of the more complex pieces of work I do for my employer, on how to navigate office politics, and so on. I really like the fact that our relationship is more than just that I love her, but that I look up to her as someone who has much wisdom, who is supremely confident and an intellectual force in her own right.
I wouldn't have that if she'd always been a stay-at-home Mum. I respect the decision of those women who do, but I value that my Mum has these other sides to her.