There appears to be a school of thought in deck building that it is worthwhile to protect the tops of joists by covering them with a waterproof barrier. That way any moisture can't sit on the top of the joists, soak in, and gradually cause rot or premature failure in pressure treated wood.
However I've also read the viewpoint that while this practice may protect the joists it can also increase drying times for the deck boards. Assuming the decking is wood this seems like a significant risk.
I would imagine that the value of this is highly dependent on climate, the wetness of the site, solar exposure, etc. But I'm assuming the site is wet or else this wouldn't seem to matter one way or the other.
Also, though a cynical point of view is that this idea is driven by product company marketing strategy; but that itself doesn't mean it has no merit.
Is there yet any industry consensus on practices in this area? For example: building codes, studies being done as decks have aged, etc.? I'm open to global perspectives although I'm in the US myself.