Honestly, TotenMitMacht, Nightfall is a concept album based on J.R.R. Tolkien's "The Silmarillion", so they felt that those screams would enhance the album, the same way that the sounds of the Ocean do on "On the Breath of Posiden" by Sym X...It just so happens that there are more of them, but then I guess that Blind Guardian just wanted a more theater/operatic-style than Sym X...yeah, the sound effects and stuff just help give the story more "life". It is just something that some people dislike more than others...
I do agree with you about Rhapsody going WAY overboard on the fantasy storytelling, though...I still LOVE Rhapsody, but wish they could do without the whole minutes of Narration....