Congrats on baby #2!! Also, is it weird that I always though it was brip-brap until I saw the blap in this post? This is where speed reading really fails me.
I read a book on this topic, The Feminine Mistake. It discusses exactly what your wife is hearing - how bad it is to stay at home, and how not working affects the mother and child. Kind of one-sided, but it touches upon some valid points.
This is such a touchy subject, probably because raising your kids is such a personal issue and nobody wants to hear they're doing it the "wrong" way. It's not really a one-size-fits-all scenario, and finances aren't the only thing to take into consideration.
That being said, I think the "we need two incomes" defense you hear from friends is a load of bull. They should man up and acknowledge that they want the lifestyle two incomes provide. To judge others because they made a choice and others made a different choice is pretty lousy, and I'm sorry your wife has to hear it all the time.
As for myself, I have NO IDEA what I'll do when we decide to have kids. Probably try and find some kind of part-time working balance.