Well it was 6+ in the original Catachan codex, before they got smushed into the new dex with the rest of the Guard. They also wear Flak vests and presumably weaved into their combats as well.
Marines however wear a body glove which is how I interpretted it. A 6+ save could be warranted I suppose if you argue those Bodygloves contain a flak weave in them, but why bother when you usually wear power armour?
The point I'm trying to make is that although their will be differences in capabilitys with and without armour, its not significant enough to reduce a toughness. Look at Sisters, Power armour and T3 they won't have the buffs and combat stims of astartes armour, but it doesn't drasticly change their performance, If we assumed they were given it would you be happy for them to suddenly get T4?
As for no armour on Marines, what purpose does it serve, no armour is exactly that, no armour. Toughness bonusses and solid rib cage and super clotting blood are all taken into account with T4. Lacking even the squig skin shirts or flak vests. You obviously want to have them as a rescue objective, so having a save is a bit pointless.