For Gilneas

The initial idea behind the attack on Teldrassil was to create a political crisis within the Alliance. The Alliance has always fostered a remarkable level of unity - while Garrosh was banning Trolls and Goblin from the center of Orgrimmar (respected heroes excepted, of course) Varian was having his son study with Velen. What internal strife there has been has generally not shown up in-game - we never saw Moira try to take over Ironforge, for example.

In "A Good War," we actually get a sense of how clever the strategy was: The Night Elves are the most independent of the Alliance cultures, and one with a certain bearing of history on Azeroth (Draenei culture is over twice as old, but their refugee status has humbled them.) By capturing Teldrassil, the Alliance's hand would be forced - Tyrande would never respect an Alliance that did not come to her aid in re-taking her capital. But given how long Gilneas has been occupied at worst and a battleground at best, the theory went that the Gilneans would resent the Alliance pulling out all the stops to liberate Teldrassil when Gilneas is not free. The idea then, would be that Anduin would have to sacrifice either the support of the Night Elves or the Worgen/Gilnean humans and thus be exposed when the Horde marched on Stormwind.

It's not flawless, though. Given how generous the Night Elves were to the Gilneans after the Forsaken invasion, I could imagine Genn putting his full support behind such an effort, and indeed, there are plenty of Gilneans who would need rescuing if Teldrassil were occupied, so I think that this fracture would not be as strong as the Horde leadership assumes it would be.

Still, it's moot.

Burning Teldrassil as a tactical decision accomplished the goal of yes, removing an Alliance stronghold from Kalimdor, and its intention was to be so brazen and overwhelming a strike as to intimidate the Alliance, force them to reconsider fighting the Horde for fear of falling to a similar fate.

Except that this second reason also doesn't work, because the Horde were the aggressors here. What had the Alliance done except disrupt azerite mining in Silithus - a typically limited engagement? Indeed, what it really told the Alliance was that they have to go on the offensive. And that was also part of the plan - Undercity was rigged as a big trap for the Alliance, but it also wound up very costly for the Horde, so... not a great win? (Especially since Alliance losses were not as devastating as Sylvanas had planned for.)

But something I wonder if Sylvanas had forgotten to consider was that by removing Teldrassil from the map - both as an Alliance stronghold, yes, but also as a liability that required liberation - she opened up the Eastern Kingdoms to Alliance domination.

Frankly, at this point I don't really see why the Horde would even want Gilneas anymore. It was meant to serve as a port for Undercity, but with Undercity gone, it would seem more logical to focus efforts on Quel'thalas (an area that I think is super-ripe for conflict given that a sizable portion of its population is now with the Alliance.)

But the Alliance has both the opportunity and the motivation to take Gilneas back. Indeed, the main reason for the Horde to try to hold it at this point I think would be to deny it to the Alliance. Burning Gilneas City to the ground would probably be a more reasonable tactical move than Teldrassil.

The Alliance does not hold Lordaeron City - no one does at this point - but the Forsaken must be barely holding onto their territories in Lordaeron. For Genn, it's time to come home. Hell, even the Kul Tirans might be invested in reclaiming Gilneas, given that their ancestors came from there. Indeed, the Gilneans might pay the Night Elves back by inviting them to live with them in Gilneas (and they could combine druidic powers to cleanse the place of plague.)

On top of all that, at a meta level, beyond phased quests that end in the teens, Gilneas is empty, doing nothing. It's a gorgeous zone (Gilneas City, while rather small, is really cool,) and I think it would be great to return there.

So perhaps we'll see it in a later patch?