It's not a question of whether they should exist at all but of whether they should infiltrate the collections hierarchy to the extent that someone has to maintain code which looks like this. And there's plenty of it: this is only an example.
trait ParIterableViewLike[+T,
+Coll <: Parallel,
+CollSeq,
+This <: ParIterableView[T, Coll, CollSeq] with ParIterableViewLike[T, Coll, CollSeq, This, ThisSeq],
+ThisSeq <: IterableView[T, CollSeq] with IterableViewLike[T, CollSeq, ThisSeq]]
extends GenIterableView[T, Coll]
with GenIterableViewLike[T, Coll, This]
with ParIterable[T]
with ParIterableLike[T, This, ThisSeq]