If you change the owner of a My Site Site Collection for a user's (User A) My Site you will, if you put yourself as the owner, be able to administer that user's My Site, but it will also have a very nasty side effect:
If someone click on the User A in, for example, a contact web part they will be taken to YOUR MY SITE and NOT User A's My Site.
Why would i change the owner of User A's My Site in the first place? To be able to have more control and reduce the operations available to User A in his/her My Site.
This is, I guess, by design, but to me it seems to be a strange combination regular program funcionality and security ....