1) eventually, the population boost will be capped, and keep in mind, disbanding orcs to do as settlers is basically you can do this kind of things in game, but unless you can convert orcs into good humans again ...

. And orc buildings reduce health I remember, your high population can drop to mere hundreds if plaque struck. And your general who stationed there can die so sudden.
2) well, and disable armour/weapon upgrade? err... just no then... I prefer having slower to train mannish troops with silver weapon or armour rather than orcs who die like flies against haradrim missile spam
3) Actually, Trolls are the most cost effective when you also comparing their battle performance, as long as they're supported and enemy line enggage yours, tell them to run and attack, then you can see them play golf with the enemy troops as balls.

1 unit of Olog Hai should usually rack arround 400-700 kills per battle if they're properly used. Just watch out for their rather fragile morale. And... actually, the best support for them comes from mannish troops who's more reliable to hold the line while your trolls smashing them to the sky.
4) you obviously never facing another terror units (berserkers, far harad men, king's horsemen, etc), they'll merely run on contact, while most of your mannish troops still keep fighting. Well, of course, if properly used, Orc Champs are killing machine, but when they're not on the mood...
5) true, and at least they have arrows, and you won't need much of them, that's why minas ithil alone is sufficient (they're 0 turn production!)
6) and warg is on Rhovanion land, yeah, they're useful, but warg archers only, the plain warg are pretty much no use