And yet unfortunately, that is probably what will happen. It is stupid and petty that when a new character comes on the writers seem to feel it is easier to write for the new character b/c he/she is "new" and in doing so they ignore the older characters. I personally feel it is b/c, as I said, easier to write for something new than something old. It is a cop out. This is a "cast" show...if it wasn't it would be called the name of whoever is the main star and I am tired of some characters being ignored. They (the writers) don't have to....they just need to make an effort in order to avoid it. And the show could go on without a new character being brought in. The cast is pretty crowded as it is and like you said, they only do one case per ep unlike the other two. And I know it is tv, but honestly, they take the unbelievable to a whole new level by having the team solve a case in ONE day.
Name a tv show that hasn't done it with a new character? LV did it last season with Langston and are seemingly going to do it again. I think it is a rarity when a the new character in town boosts the ratings for a show. NCIS and Cote De Pablo is one of them. She is great actress with a great range and the writers for that show are far superior to ours.
But not many shows with main characters changes have seen growth in their success/ratings....I mean if there have been more where a replacement has made the show better in the ratings category do name them. I am not opposed to eating some humble pie every now and again.