If Jobs will only have the abilities of their corresponding Class and Subclass, it would seem that the standard Class would have an advantage over the Jobs since they are able to use abilities from all other classes.

If we compare MRD against WAR as a DD I'm likely to view MRD as the better choice.

MRD can use:
Blood for Blood
Invigorate
Keen Flurry
Quelling Strike
Blindside
Hawk's Eye
Incapacitation WSs (Demolish, Leaden Arrow, Gloom Arrow, Heavy Thrust, Impulse Drive)

WAR gets:
Double Attack
Counter Attack
AoE Attack Up buff

Now if those 3 abilities makes WAR a better DD than MRD I'd be the happiest Highlander in Eorzea, but I'm very doubtful of that. And I'm willing to bet those abilities have a 5 min+ recast timer on them.

Even when it comes to tanking I still don't see WAR as a winner over MRD.

MRD can use:
Cure
Sacred Prism
Stoneskin
Protect
Second Wind
Featherfoot
Sanguine Rite
Heavy Thrust(stuns and incapacitates legs)
Raise (Not really needed for tanking, but nice to have)

I've heard some say that the Jobs will only have the abilities of their Subjob up to level 15, and if that's the case MRD will have Sentinel and WAR won't.

So if WAR turns out to be worse than MRD at tanking, who would want a WAR over a Paladin? And if MRD can do more dps than WAR, who would invite a WAR over a Dragoon?

Hopefully I'm missing something huge and this is not the case at all. Perhaps WAR will get an incredible stat boost and be "the most fierce fighter on the battle field" as SE describes them. And I'll be able to put my fears of never getting my WAR a party invite to rest.