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In yer opinionz, iz it worth givin' a Nob in a boyz mob (like a Slugga or Shoota mob) eavy armour fer 8 pointz?
I alwayz sez no (like dis: "no!"), 'cuz uv how diff'rent armour saves iz figured out, but iz it actually worth thinkin' about?
In general I don't bother with heavy armor upgrades for Nobz in ordinary mobs for the very reason you mention; it never really benefits you until the Nob is all by himself and at that point a 4+ armor save probably isn't going to make a difference anyways; I'd rather buy another boy with the points.
The bigger problem of course is that most weapons are AP4 or better so in many cases the 'eavy armor is a complete waste of points. Even in CC it doesn't make a whole lot of difference because: A) whipy CC armies aren't a threat anyways & Tough armies (e.g., a squad of terminators) is going to be armed with weapons that nullify the armor as well.
About the only place I might take the armor upgrade is in my Warboss's body guard mob (if I use one) and then only when I'm going up against an opponent where the armor might make an actual difference.
NAR! spend'in da 8 teef on unuver 'o' da boys yah get more dakka!
I take eavy armor in a nob retinue, or in a CC specialist unit...like my Skarboyz.
In regular mobs of boyz or tankbustas then no, any shooty mob will not benefit from the save unless the majority have armor. The armor really only helps in CC.
WAAAGH! Da Eavy armored Nobs
"Da Orks is da best cuz we ar ded ard an killy, mean, green, krumpin machines, hav da most dakka, da stompiest dreads, da shootiest gunz, da biggest bosses, an we uze choppas...WAAAGH!"