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Is taking a plague furnace and a screaming bell a waste of points?

Not really. They both get you nice unbreakable units, a good Ld bubble range of 18", and the furnace is a good combat unit

The bell is pretty good too.
I've run huge blocks of storm vermin with the bell, and they can put the beat down.
I take the triple march banner, hold back turn 1, then march 15" wheeling into a flank. Then I ring the bell on 1 die, and more often than not, it moved D6" forward into a flank, completely crushing the unit. (3 ranks, standard, charge, flank, impact hits, and a good number of WS4 S4 attacks).
Since the unit is unbreakable, I'm not too worried about getting hit in the flank, I just reform in horde formation facing them and start the grind.
Combine that with the plague lore, and it does pretty well.
-Matt