I think the worst side-effect of this is that you can't send another friend request if you accidentally delete said person off your friend list. Well, you can, but your friend won't receive said request because you're still on their friend list. =/ (Same if you or they move to a different server and then change minds and move back.) Not unless the friendship is "ended" from both sides first. And sometimes, especially if one side of the friendship is slightly OCD, that's tricky. (I.e. friends are listed in order of being added, meaning if my very first Eorzean friend were to unfriend me for some reason, and then wanting to be friends again, if I remove said friend they'd no longer be counted as my first Eorzean friend, but rather my latest. And while stuff like that might not be too important to some people, it can be very important to others =/ )

You'd think that if you send a friend request to someone who has you on their list already, the request would be auto-accepted, but no. For some reason it gets auto-declined instead ^^;