I suppose if you have two separate member lists (let's say, for example, an offline and online list), you might want to make sure that both lists used the same (probably the earliest) registered date, as this is something that shows on a user's profile page.
I have an email list which I am merging with the forum. The forum was the older community (on VB).
Some members show on both but there are lots on the email list whose Clan join dates go back a while. Yet others are offline members of our House Clan who I am coaxing into the online community so everyone can connect in one place.
I wanted all their varying join dates to be accurate in the new merged community.