If i get it at all, I usually get a troll cage much earlier, and then I build my strategy around that. Meaning that I go for troll discount, for example. Trolls do give you harassment potential, that you normally have none of with Mordor. This allows a more aggressive playstyle and usually works best vs Rohan and in a mirror. Gondor and the dwarves are very strong defensive factions, which makes harassment harder, while Isengard has easy access to archers with their scouts.
This strategy does hurt your economy compared to an orc spam, which means that your first outpost will be delayed, which is quite risky. So you have to make those trolls count. If their impact isn't great enough, you're usually too far behind to win the game. At the point where bigger numbers of archers and herokillers (btw, I'm totally ok with them doing a great job vs trolls since they are single entities) hit the field, trolls don't do very well any longer, so if you are missing an outpost at that point, you're in trouble.
TL;DR: I use trolls early on, with upgrades they scale into the midgame. After that, you need leveled heroes and outpost units.
