I'm a full time SAHM, and I have Nina in daycare 2 times a week. I initially did this when she was 6 months old as I work went back to work on those days. But I'm not working at the moment, and I'm not sure if I'm going back, but I have kept Nina in. The main reasons I keep Nina are these:
~ socialising. We have a few friends with other toddlers/kids which we know, but we don't get a chance to see them often as they work.
~ I need a break to catch up on the housework, errands etc.
~ It gives me one on one time with Emily. As I get this one on one time Emily during the week, on the weekends, I make sure I give Nina one on one time as well.

So they are my reasons. It's a very personal choice. Make sure you are completely happy with the decision you make, and make it by yourself. Don't let others POV affect your decision. It's not their life, it's yours. You need to do what's best for you.