I go to Blasted Lands to farm Rugged Leather. The mobs also drop imperfect draenethyst fragment which starts a repeatable quest that rewards greens which can be good transmogs. So I go to Blasted Lands a lot, and the quickest way for me to get there is to port from Org to Shat, fly to Hellfire Peninsula, and go through the Dark Portal. This is also how I get to points north, such as Blackrock Mountain.
There are other ways for Horde to get to the lower Eastern Kingdoms (zeppelin to Stranglethorn, portal to Twilight Highlands) but I find the Dark Portal very convenient.
Going from Hellfire Peninsula through the Dark Portal to Blasted Lands is working fine for me (thankfully, since I use it regularly). Going from Blasted Lands to Hellfire Peninsula, the Dark Portal doesn’t work for me, like so many others have reported.
If I’ve had Zidormi put me in the present time, the Dark Portal takes me to Warspear. If Zidormi puts me in the past, the Dark Portal isn’t a portal.