I generally build armies taking into account model variety so I do not see the regular orcs outdated. And there are equipment that the Morannon cannot get and "regulars" can, like bows (albeit everybody will use them as trackers, no problem) or two-handed weapons.
I agree that the orcs come in different breeds and the Morannons are well into the believable.. they are just the biggest and baddest of the regulars, and they bothered to equip them with slightly better stuff.
Is cool that they are only in the Mordor list, and other evil factions have to use the regular ones (barring Isengard and its Uruk-hai)... is very in-character for Sauron having kept the "best" orcs for himself. S4 is big, but we have seen such power creep in other armies too, at least as an option unblocked by a character.
Furthermore, we know Saruman had orcs, uruk-hai but had other half-breed creatures as well, and there is evidence that the uruk-hai were not such hybrids. So it makes sense that Sauron, powerful as he is, managed to find a way to "build" or "select" bigger orcs without going into the Uruk-hai territory.
The Black Guard, that one is a bit too much for me. Great models but is just beyond what I expect an orc, even an uruk-hai, should be able to do.