I don't think that Noteboat is saying that there aren't other reasons for using slash chords, such as pedal points. He gave decending bass lines as one example of when they would be usefull.
He is saying that most of the time they're not necessary, and in that he's right.
Kingpantzer, this is exactly my point. I think that in 99% of written slash chords they are necessary. The slashchords almost always demand a decending/ascending bassline or a pedalpoint. If you can give me an example of unnessery use I would like to see it!
When I write music and write slashchords and someone in the rhythm section (piano/guitar) is not taking it into account I can immediately hear it, especially when the third is in the bass!
I am not saying that when a guitarplayer voices the fifth in the bass it is slashchord perse, because of course guitarplayers tend to do that. That causes no real problem as long as the bass is playing the root. For example when you would be playing in a bigband comping Freddie Green style (using only the bottom four strings) and you play:
G7 C
you'd probably play:
G7 -> 3X34XX
C -> 3X25XX
Actually you'd have to voice the C chord that way because it is the only right voicing to follow that voicing of G7 when taking voiceleading into account. The C chord has a "g" in the bass but I would not consider it a slashchord.
However, when I write:
G7/B C
I'd expect the guitar player to play:
G7/B -> 7X57XX
C -> 8X59XX
