Obviously, the child would benefit from having a mother and a father. However, due to the failure rate of Hollywood marriages, I think the couple should only get married if they truly love each other. Otherwise it will just be another Hollywood divorce in a year. Their daughter should have the most comfortable environment possible, which would be in a home with two loving parents.
Call me old fashioned, but when you have a child together you should generally be married. Unless there are irreconcilable differences, marriage is the best option for the child. Since they are still a couple and have the mutual commitment of a child, they have no good reason to remain single.
This man and woman should marry. If they already had a child, the child would do best in a home with two caring parents. They should have thought about the possibility of having kids before they became intimate. This is an example of why society is not getting any better.
I think they're rich enough to where the legal benefits of getting married wouldn't make much of a difference, and could actually just complicate how their assets are distributed. If there relationship ever fails like most do, even after their children are grown and gone, being able to leave and move on could take years to shift out legally. I personally think it's smarter to not be married but to stay in a committed relationship. I don't think the child would benefit from a marriage anymore than a stable, loving relationship.
I guess I have never even heard of these people so I am questioning the level of their fame. With that being said their fame should have nothing to do with if they get married. Unless they want to have a 2 year marriage and a handful of headlines. Get married because you want to be married to them not to improve an image.