Outside the starting zone, they're too few and far between. 1 tree in darnassus, 1 in felwood, a tiny section of blasted land - and a handful of npcs here and there, hardly worthy of race.
Why not turn Theramore humans into Worgen? Theramore gets destroyed, the worgen curse spreads through the human and high elven populace there like wildfire while Jaina is in Stormwind visiting Varian. NElves & Gilnean Worgen come to the rescue, and now the humans of Kalimdor are now Worgen.
Theramore rebuilt to look like Gilneas City, and the alliance assault into southern barrens is the counter-attack. This way you get a far greater presence of Worgen in the alliance. The fact that Theramore worgen are from Lordearon makes the forsaken undead hate thing deepen, and now they team up with the Gilneans, they're all worgen now.
IDC what they do, just more involved worgen about. It makes sense to have more worgen around in Kalimdor anyway, and they're part human anyway, so why not have them be the human related wing in Kalimdor? You still move from Gilneas to Darnassus as a player Worgen, but by the time you reach Dustswallow marsh, the said events have already happened...it's too late to introduced phased quests about it, it's just one of the changes that happened in the aftermath of the cataclysm after the events of Gilneas.