1. Because they're not popular and most survival abilities cross with the need to keep agro cannonwise. Holy build is just better and it has great damage output because of Determination stat.
2. You can be an offtank (if the objective is to survive for ~15 - 20 seconds) with even a dps build if you have basic barrier rotation skills and challenge skill.
THIS build should work for it.
3. If you
really want to try physical tank, you can try something like
THIS build. Righteous Word (later - RW) and Challenge helps you hold agro much easier. (Without RW your agro is non existant.)
Besides, these "Finesse" stats in the first ruby grid are actually "Determination" and they are a must-have as tank. I suggest you aim for 40 - 50 of those.
4. It's because they can tank, spank and even heal with the right choice of build!
5.
THIS would be a better choice of talents and rubies when you get more of them.
Protection of Faith rubies in the third grid help you rotate barriers easier. It's because rotating barriers is the most important thing for any type of paladin tanks. Departing Light and selfheal rubies in the bottom of third grid greatly help your heal keep you alive in astral. As long with 12 energy bonus for every barrier broken (Overcome breaks a barrier too so you can manually gain 12 energy). And this is why I suggest you take
Departing Light rubies instead of Vitality (2nd grid) for energy gain. They work out better than passive 2 energy every second during the fight because as a tank you should pretty much always have enemies attacking you.
Hope this helps, cheers