Oh, yeah, I agree completely, it's a necessary part of the story - and there was a real need to wrap up Londo's story properly. (If nothing else, season four alone doesn't really establish that "War Without End" is still going to happen...)

(I do wonder if I'd enjoy this part of the show more if we got more of a sense of the Drakh as individual villains - as it is, it's just 'horrible things are happening to everyone', whereas at least with the Shadow War, you could point to Mordin smiling away and think "Well, at least someone is enjoying themselves... *Grin*)