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I would have guessed it was for the derailleur cable but maybe I'm looking at a different spot to you.
But yeah spare spoke holders are awesome. Want to weld one on my frame but really it's not going to work well unless each set of spokes (NDS/DS/front/rear) is a very similar length as they'll just rattle around if they're not taut. Also the spokes aren't going to fit when you swap wheels.
Something adjustable on a rail would work well but it's a bit more complicated than is really justified...
That comes with a spare spoke carrier on the chainstay? Rad.