Well first off, it's a penninsula, meaning that it gives them a better water route. If the alliance hold it, then the alliance have a much easier time attacking the forsaken. Even if it's only marginally more useful than silverpine for the horde, it's immensely useful for the alliance, giving them a very close access point to silverpine, which is only a stone's throw from trisfal.

And the horde went in there wanting the territory, simply. It's a war, just like any other group would do, if you invade an area and they don't surrender, then you take it by force.

And I don't know how much Garrosh had to do with Sylvanas actually intruding Gilneas, he obviously wanted the land, but Sylvanas is obviously not the type of person to take mercy on people.

In a war, if you want an area of land, and the people aren't willing to give it up, you attempt to remove them. It's not the nicest thing to do, but there is a difference between honor and being nice. You can be an honorable warrior but not be nice. They fact was, they wanted the land, the Gilneans didn't retreat (quite the opposite actually), so the horde attacked and attempted to take it by force. They eventually retreated.

While it's generally a good idea to leave neutral parties out of wars, if you want an area bad enough, you'll take it by force if necessary. That is part of war. If the Dragonmaw decided to not join the horde, they'd probably all be wiped out now, or at least wiped out to the point where the remaining members joined the horde (basically what they did.)

War isn't pretty, but that doesn't mean you can't attack areas without honor. The reason the attack on the druids lacked honor was because it had no real significance to them.