Nostalrius Begins - Quality wow vanilla realm (1.12)

Im leveling a 2h Fury spec and have not taken Piercing Howl, I prefer to tank the dungeons of my level and someone else in the forum mentioned Piercing Howl for a tanking spec, I can understand preventing alot of stuff happening from runners, but does it cause much in the way of threat?

I'm no expert on the subject but I would assume it does as much threat as demoralizing shout as both apply debuffs. Regardless it's a good ability to have for many situations. There are times where I coulda lived if I had the aoe slow as I tend to hamstring targets and kite if the healer is having trouble keeping me up for whatever reason. Even if you have shaman, mages, etc you can't always rely on pugs to think beyond their own dps.

Piercing howl does a bit less threat than demo shout.

There are several benefits of having Piercing for tanking. As Daniel already mentioned, it's great for kiting when you take a lot of damage. It's also really nice for situations where range dps go aoe ham before you have a chance to hit all mobs. The slowed mobs that turn for ranged dps are much easier to select and get aggro back on before they run out of range. Additionally, it provides a slow when an AoE class doesn't have one (lock, hunter (most don't use trap anyway), druid).