More anecdotal "evidence" here...
Before I had children, and when my first was a baby I agreed with your dh. Mainly because of my own experience of having a sahm and finding starting school, being away from her for the first time, incredibly traumatic. I wanted my children to be secure with being left and get used to the fact that I always come back for them before they were old enough to know different.
DS started nursery at 8 months - only a couple of half days. Built up slowly to 3 days by the time he was 2. It was fine for him but I can't say that he developed social skills from it. He enjoyed it though and has happy memories of nursery.
DD1 started nursery at 13 months - it was much more difficult to leave her because of breastfeeding, she never took expressed milk and was much clingier to me than DS. She hated nursery to the point where i thought I'd have to give up work...then she settled and she too has happy memories of nursery. She is not a confident child though, doesn't go into new situations easily and still (starting school in September, about to turn 5), prefers to be with me and misses me when apart. She socialises well in small groups of people she knows well but just isn't a big group person.
DD2 has never been to nursery, we just couldn't afford it and I am now a SAHM (was part time until her birth). She is 2 1/2. I'm about to send her to playschool 2 mornings a week just for a couple of hours. She socialises well at groups with me, doesn't cling to me, goes off and plays. She is keen to go to playschool, she actually asked to go, and I think she understands that I won't stay.
I think it is personality more than anything else but having read more about attachment theory (google, you might find something to show dh), I do believe that children are most secure when allowed to separate in their own time. I don't think dc4 will go to nursery either.