I must admit have said those words. Not in such a horrid way, more in response to people who tell me (in a slightly judgmental way) they need to just love their child and spend time with them before they have another. I think that motherhood has taught me that a human's capacity for love is not finite. Each new person that enters your love world just increases your capacity to love.
I am in awe of the sibling relationship. The way my kids reserve their extra big smiles just for each other, the way my son watches my daughters every move, in awe and wonder, learning. It is more amazing than i could have imagined and I hope it stays that way for a long time.

But if I really thought about, of course it is not the best thing you can do for your children.
Mother Theresa said the best thing a father can do for his children is to love their mother. I think this is true. If children are in loving environment and learn respect for themselves and other, that is the best thing you can do for them.

If I was having a cup of tea with you and you were pondering whether you should have another, and it seemed like a possible thing, something you could do, I would encourage you to. If over a cuppa you said that you really couldn't do it all againi and DD was your only one, I would support that too, you will build your community around you and she will have go to people. You need to do whats right for you ultimately.