Is the antiquity of a text a valid reason to reference it in a discussion?
Can well be. For example, if we were discussing what people of the early bronze age thought about a subject then texts from that period certainly would be appropriate.
Are holy texts useful when discussions include non-adherents?
Again, it depends on the discussion and on the content of the text. The content may well be useful regardless of whether or not someone accords them some religious status.
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