A one-year old doesn't need to go to nursery for social development, but there are various circumstances where it may be necessary (work, no family nearby, other commitments of parents where it's better for everyone else (including the child) for the child not to be there).
I went back to work when my DS1 was 15 months, and he went to nursery. In hindsight, it was too young for him.
When he was about 11 months older, so just over two, I finished working, and still kept him in one day a week (I had lots of medical appointments not close to where we live and neither of us could have coped with carting him along!), and by that age he loved it - I knew I had made the decision when he started running into nursery in the morning!
Now that DS2 is here, and he is older, he goes to nursery a fair bit now, and he loves it. He tells me on the days he is home that he wants to go to nursery.
Please don't be so harsh on mothers who don't have other means to look after their children. I don't have family nearby, and my DS2 has his own issues, so the only option is for me to put DS1 in nursery. He is happy, and I get to meet my DS2's needs as well.
As an aside, when DS1 was starting at nursery, I met another mum who was SAHM who had her young baby in nursery for 2 days a week. I didn't agree with it at the time (why would a SAHM need a nursery?) but now I can see that some time apart from her son meant that she was actually a better mum, and once we get DS2s issues sorted out I will probably send him for 2 mornings or afternoons a week).
It is a personal decision, all our circumstances are different, and just because we wouldn't do something because our circumstances allow, doesn't mean those same actions by someone else who has no other option is "wrong".
I don't know why we are so judgmental of other mums!