Conflict can be destructive or it can be productive. As well, the tunnel vision that comes with a single perspective, not having someone to balance you off, not having someone to share the load, has far worse impacts than conflict.
Statistically speaking, it is still overwhelmingly clear that the best results for child rearing come from dual parent homes.
Single parenting is such a hassle because all of the responsibility for child care is on the parent. With a joint partnership it is possible for one partner can take a break and catch a breather while the other partner occupies the child's time. The conflicts are simply the drawing out of the best way to parent the child, through multiple perspectives.
While single parenting avoids the conflicts you get in marriage I think single parenting has other issues that joint parenting does not, I think it's healthy for a child to have both a mother and a father figure in their lives teaching them their separate ways, you can't really get that with a single parent.
There are plenty of single parents that are successful and do right by their children, but that doesn't make their situation preferable to having two parents in a household. Conflicts in marriage happen, sure, but in any healthy marriage they're not even close to offsetting the value of two parents in the house.
Joint parenting is preferable. Even if there is constructive marital conflict (abuse not considered constructive) this is healthy for a child to witness. The child will then have direct access to seeing conflict resolution. Not being exposed to healthy interactions between parents, the child must resort to external sources such as television or video games to model their resolution tools.
Single parenting is not actually preferable to joint parenting since it avoids the conflicts that arise in marriages. It always helps to have two parents in the household so that one does not always have to parent. I think single parent homes work fine if there is enough time to parent.