Spelling out the future evnts of a character can give a story structure and prevent it from careening off into the wild black yonder, like nBSG did. On the other hand, it can really limit the writers and prevent them from taking the story in interesting directions. As a constraint it is a double-edged sword.
For example, Valkyrie was a good movie, but it was ultimately harmed by the fact that they felt that they had to keep Hitler alive. The problem wasn't the spoilers, though. It was the fact that keeping Hitler alive ruined the climax and destroyed any emotional catharsis that the movie had built up to that point.
In Londo's case, though, I felt that his fate was satisfying, given everything that came before. There wasn't any other way that it could have gone, really.