Why everyone wants Nico Robin?

Her special gives 1.5 Int damage for ONE TURN, so? Buggy captain gives 2x Int attack EVERY TURN. I'm confused.

It's because the multiplier, well, multiplies the damage. Say you have the means to change all your orbs to int with special move orb changers; that, plus current Buggy captain ability, gives you 2x2x2 attack power for int. First form of Nico Robin gives 1.5x that for a total damage multiplier of 12x. Her second form multiplies it by 2x, so you get 16x damage.
If you wait until towel Nami is released, the formula is 2.5x2.5x2x2 which equals a 25x multiplier to your damage output on one turn. If you're ko'ing a boss that does horrible things at low health on the turn before he wipes you out, it's very good. It can be very powerful.

I almost forgot about the perfect multiplier for that turn. The damage will really start skyrocketing. Doom hit with all perfect hits beforehand will be a further multiplier of 2.5x. The total multiplier for the last hit will be 62.5x. That's scary high.

Is it worth leveling of Nico right now? I have her, but I rarely have any Int characters. What's a good team to build around her?

My best guess would be that current INT teams would be

Captain Buggy
Thunder Tempo Nami
Nico Robin
Iron Wax Mr 3
Captain Krieg MH5
Golden Pound Usopp

At least I think Buggy would be captain for the 2x damage instead of 1.5x Nami gives though she does give more survivability with 1.5x health too