Feralas is definitely worth it for them in my opinion. Not only is that the base of their sentinel army, but it's a pretty much untamed wildland that would be hard to fight in. On top of that, it has a major Shen'dralar city - Dire Maul - and contains one of the gateways into the Emerald Dream.
Feralas is basically the only zone left in Kalimdor in which the past, present and future of the night elves collide, from every single aspect of their society at that(ie Shen'dralar mages, the base of the sentinel army, the Emerald Dream's gateway which druids are no doubt interested in).
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Are we certain that the Forsaken hold Shadowfang Keep? I believe that the Bloodfang pack was the last force to enter and claim Shadowfang through force.