[Farael]

I honestly do not see where you find that the Nazghul would not have been inclined towards using subtler tactics. I think that the Witch King was a very competent military leader and thus he would have used the best tactic according to the situation. If he did not have enough forces and supplies for a conventional siege, I'm sure he would have opted for something else.

If you are thinking about either the fall of Arnor or the Siege of Minas Tirith later on (which I'm sure we all agree WAS a conventional siege, until broken) in both cases the WK had superiority in numbers and he was fighting a disheartened, weakened foe. While the garrison in Minas Ithil was certainly weak and disheartened, Minas Tirith and Osgilliath were close enough that he knew he would have had to deal with them. Therefore, it's completely plausible that he chose an alternative method.

Furthermore, with regards to your comments on supplies for guerrilla warfare, they did not need to win the love of the Haradim or other evil men as long as they could scare them into doing what they wanted. And I'm sure both orc and Nazghul can be rather scary.