There is another concern that applies to both importing gedcoms and also using an existing profile when extending a family tree.
If I create a new profile, or import a profile through a gedcom and THEN merge it, the new profile is on my watchlist, and I am a manager of it, and the profile is part of "my" family tree. If on the other hand, I use the existing profile, I do not have the profile as part of "my" tree unless I get permission.
I must admit that I find the process of getting permission for each ancestor or line to be cumbersome, and without it people in my direct line are not on my watchlist.
Also, I think I have seen some 'gaming' going on whereby someone creates a new profile, and then merges it to an existing profile in order to become part of the trusted list, and thereby a manager of the merged profile (also bringing in unsubstantiated children in the process).