In my experience, and I get this only from numerous sieges, I got no code-backup on this one, if the enemy has loads of siege equipment, its easy to burn down one or two of them. If they have only one siege equipment, its damn near impossible to burn down.
Anyway Ithilien rangers: Let me guess, you put them in front of the infantry while the gondor archers are behind? If you do that, dont. In any case its not that Ithilien rangers are worse, its how you use them.
As for generals, keep them in the towns. They provide population growth which ultimately aids your cause. (Unless he's a squalid corrupt

, in which case send him on a one-man expedition to mordor)
Unfortunately Osgiliath and Moria is pretty much impossible to rebuild without cheats so dont waste 'em there. (Or at least I tend to finish the game before getting halfway)