It's the +10 Maximum Lightning Resist that makes them so good. A character's Belt is usually the least important piece of equipment to a build, ergo, TGods is highly useful. Think about it this way:

10K Lightning WITH TGods (not counting the +20 absorb, since there's some debate as to exactly how it works):

10000 / 4 = 2500 (pvp penalty)

2500 * 0.15 = 375

375 is what you would take.


10K Lightning WITHOUT TGods:

10000 / 4 = 2500 (pvp penalty)

2500 * 0.25 = 625

625 is how much damage you would take


Of course, these are both assuming you're running Max lightning resist.
That should be a clear enough example why TGods is good. After the absorb is counted in, you'll be taking even less.