Sorry but I was just talking a contractor here about this and he says that silicone will not reliably stick to old silicone in typical applications such as bath tubs and showers. As a result you must always remove the old stuff first. This however does not mean actual silicone will not stick to silicone if it's clean. The problem again comes down to reliably cleaning it which isn't always possible and therefor any manufacturer of silicone advises that silicone won't stick to silicone which means the old must be removed, this is also often easier than cleaning the old stuff and you get a nicer looking seal.
So that's where the myth comes from, not so much a myth I guess but more like a better safe than sorry kind of thing.
Sorry for any confusion from these posts but I do still believe you can safely get away with only sealing the verticals.
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