The marriage dates are quite well hidden in the current geni user interface I think. It's only shown on the edit pages, which means many do not bother adding it at all.
I think it's quite good to have especially with multiple marriages and children with many partners are present in connection with profiles.
If the marriage date is there it improves the possibility for sanity-checking of childrens' parents, although it is of course not a bullet proof way of checking (like many other types of sanity-checking).
It could be shown on the main page of each profile together with the partners' name at least.
Just an opinion..