If you're not familiar with our defensive stats, here is a previous post of mine explaining what each of them does. I recommend you read that first, if you aren't familiar with them.
To summarize the information for you:
There are four ATTACK TYPES: Melee, Ranged, Force and Tech.
You can only shield and defend against MELEE and RANGED attacks. Every class in the game has at least one Force or Tech attack.
Some classes are more reliant on Melee and Ranged attacks than others: for example, Immortal and Vengeance Juggernauts; Annihilation and Carnage Marauders; Marksmanship Snipers (and their Republic mirror classes).
There are four DAMAGE TYPES: Elemental, Energy, Internal, and Kinetic.
The damage type does not determine if you can shield or defend against an attack, but armor reduces damage from Energy and Kinetic attacks. You can also, via skills, get Damage Reduction to specific types of damage.
Pros to wearing tanking gear:
- Tanking stats may not be amazing in PvP, but they do have an impact, allowing you to stretch survivability to the maximum
- Tanking gear has more Endurance anyways
Cons to wearing tanking gear:
- Reduced damage output
- Tanking stats (besides endurance of course) are completely useless against specific opponents, like sorcerers.
Immortal Juggernauts and Defense Guardians received a sizeable single-target damage buff in 1.2 since they can now gain access to Single Saber Mastery in Soresu Form. They also can use Vicious Throw earlier (at 30% health instead of 20%). Because of this, DPS gear is really personal preference for Juggernauts who are tanking in PvP.
Currently, Assassins and Shadows can get away with wearing a lot of DPS gear while tanking because, more than any other tank, they can choose when and whom to engage. In addition, they are naturally the highest-damage single-target tank in the game.
Powertechs and Vanguards can mix and match with a good deal of success, but again this is mostly personal preference.
Wearing actual tanking gear is worth it, if you want to make tanking your primary focus. It does give you a survivability edge.
If you're queuing solo all the time, you may do better in DPS gear.