@Projektt,
1) Obliterate should only ever be used on Cloth and Leather classes when NO healing spells or incoming heals are present on our current target or your opponent is a complete idiot/under geared.
2) Necrotic Strike should always be used on Plate and Mail targets as a standard over Obliterate as Obliterate does not do nearly as much damage against these classes. Therefor Necrotic Strike should be used as it's damage-over-time(from the its heal absorption) surpasses the damage of Obliterate. Necrotic Strike is also used on Cloth and Leather classes when a heal or incoming heal IS present on your current target. On a side note, Necrotic Strike also grants a 30% casting debuff.
3) With a good setup, one should use Obliterate on Plate and Mail classes when you know a kill will follow with the use of 1-3 closely spaced Obliterates. Also one can use Obliterate on Killing Machine procs but the damage from Frost Strike can out due an Obliterate on Plate and Mail classes, when used on a Cloth or Leather classes Obliterate will usually win.
4) Frost Death Knight's ARE Master Frost in PvP, never anything different and this is why:
A) Our best damage output reforge is mastery, 5% hit is gained with our gear, Spell Penetration is gained with the use of our gear's Blue gem slots and cloak enchant. The only other option is Critical, which is well below the damage-per-second output line when compared to Mastery.
B) As all reforges are to Mastery our only viable enchant scheme is to take Strength as our top priority(using Mastery for our feet, resilience for our chest, PvP enchants for our shoulders and helm, and the PvE enchant for our legs). Similarly we use strength as our main gem priority as well(this is to say we stack pure strength in every slot IF(and this is the same rule for every class) the slot bonus is 20+"AKA chest, helm, legs". You can use a Strength + Mastery gem for your chest and a Spell Penetration gem for your Legs(which should have been there anyways), our Helm is paired with the meta-gem(Strength + Critical effect).
HERE IS THE MAIN AND DEFINITIVE REASON:
1) Our main spell has always been Howling Blast, and since Howling Blast's damage is directly increased by our Mastery rating it is even more viable as our main spam. Next is Obliterate or Necrotic Strike, these two are used dependent on what class and armor type is currently targeted and weather or not a heal is present or incoming. Frost Fever is applied via Howling Blast(if the glyph is used), also take note that with the Icy Touch glyph we do not need to apply the Blood Plague debuff as Frost Fever does 20% more damage(which is also increased by our mastery rating, rendering Blood Plague useless and thus Obliterate is already at a disadvantage). Lastly Frost Strike is used to dump our Runic Power, and again Frost Strike is increased by our Mastery rating(but more so from Strength).
2) Obliterate gains nothing from any reforge less you go critical(which is useless), it does gain from Strength and the application of both debuff's(Frost Fever and Blood Plague), but the Howling Blast > Necrotic Strike > Frost Strike rotation's damage output DOES AND WILL exceed that of the Howling Blast > Plague Strike > Obliterate > Frost Strike rotation when you take into account the USEFUL up time of Obliterate, the up time of heals on your target, and the armor class of your target. Rendering Obliterate an "On Situation" spell, rather then a consistent main ability.
You are correct to say that Master Frost is "good" for AoE, but why anyone would want a DK to focus on AoE'n a pack is beyond me. The damage output isn't that great when you look at the damage per extra mob ((0.5 * (((1322 + 1322 / 2) + (AP * 0.44)))) Vs ((1322 + 1322 / 2) + AP * 0.44)) on your main target. Granted you can cause all targets within your cone to be hit with Frost Fever, still I would never aim to AoE, just focus your target and the splash damage will do the rest. As for using Obliterate on a 1v1 scenario? It's all situational and I have given you a detailed reasons as to why we ARE Master Frost...