Hmmm... So if I use Defog first, then attack someone with a Rocky Helmet I won't get affected by it, or the Life Orb I'm holding? Or do I have it backwards?
You won't get damaged by the opponents Rocky Helmet or take damage from your own Life Orb by using Defog.
The main draw to Defog is actually the metagame around it. The current gen 6 metagame has shifted in a way that bulky offense is the driving archtype (with hyper offense on the ladder). Because of this, suicide leads--who's only goal is to get hazards up and then die--are almost obsolete and bulkier hazard setters that can come in and set up multiple times in a match are more in favor.
If you Defog your own hazards away, it doesn't mean they are off the field for the entire match. Taunt is barely an answer to Defog users as you generally have to switch in your Taunt user to the Defog user and they will already have used Defog. Most of the best Defogers have some form of offensive pressure as well:
[email protected], MScizor, Zapdos, and Gliscor are all able to either threaten or outright kill most Pokemon that would try to Taunt them.
The only real downside to Defog is that Bisharp pseudo-blocks the move. If Bisharp comes in on a Defog, it gets a +2 boost thanks to Defiant and can then put some serious pressure on most teams. Seeing Bisharp in the preview also dissuades the use of Defog.