I would offer, rather than further dissecting the words to figure out RAI, that the 5++ wouldn't protect you against most common infantry weapons on the table top. If you run down to the store and end up fighting space marines, you'll be up against some intercessors with rapid fire during tactical doctrine with AP -2 shots. That still puts your guys at a 5+ save, or you can use your 5++ save.... whichever feels more awesome. Now there's some AP -3 infantry capable weapons out there and that's when it would matter (looking at you stalker bolt rifles during devastation doctrine) but they aren't a common threat to build your list around. Most of which are anti-vehicle weapons. The Vindicare Assassin being the exception??
I would regularly play Thousand Sons before the lockdowns and their combi-bolters were always at AP -2. The only thing that saved me was having some storm shields. I brought the Dominus Aegis a couple times and it still tore through my intercessors hiding behind it (in 8th, it was more ideal to get a tank in the aura). Specifically when using terminators in my kill team to take saves, it was because they inherently had a 2+ save and 2 wounds that they would prolong the life of my veterans. The 2 wounds on vets makes a difference but without the 2+ save, the common infantry weapons are going to tear through you the same unless you bring a/some storm shields.
Side note: Comparing our Veterans vs Primaris via the wounds is fine, and a plus for our veterans. But with new transports and units available, primaris lists can be built to be on par with pure Vet lists.
So from my experience, I would just offer that even though a save is a save, it didn't make that big of a difference in most of the games. Enough damage was done by that point that the unit was fairly crippled in terms of fighting back next turn. Perhaps Vengeance rounds could change that this time around... Bring some storm shields or bring a transport - tried and true.