There is also the question of contractual legalities of putting someone in the credits. Does that mean they have to appear in every episode? I am not that versed in entertainment law, so maybe someone can enlighten us.
Appearing in the credits means that you cannot accept any other acting work while the show is filming. You don't necessarily have to be in every ep (Charisma Carpenter was in the credits her whole stint on BtVS, but she didn't appear in every episode) but you need to be
available for every ep, in case of sudden changes or reshooting that needs to be done.
It's for this reason that Amber Benson asked to not be in the opening credits. She wanted that flexibility. There's a link to an interview Amber did explaining this in one of the Tara/Amber threads.
Now, I don't know if Andy had any other work outside of the show (the only other things I remember him being in was that one BtVS ep where he was a background character and 'Chance'), but it could explain why he wasn't added earlier.
There's also the notion that the writers just didn't know what they were going to do with Lorne. He's not an easy character to pin down, he doesn't 'fit' with the rest of the team, has no ties to them. After the bar was destroyed, there was really nothing keeping him in LA, so it would have been easy for the writers to decide one day that he was just not needed anymore, which would explain why he wasn't in the credits until S4.