1. I think that if you go Yen Sid's tower and then go into the room where Sora got his new clothes, there are five mirrors, one representing each of the Drives. If you examine the one for Anti-Form, I think it gives a description that says something like "In this mirror, you see yourself being overcome by darkness." That's the only story related thing I can think of.

2. It really sucked when I got Anti-Form against Xaldin or Demyx. I think the worst was when I got Anti-Form against Demyx followed by the "Kill 10 water clones in 10 seconds" challenge. No Wild Dance RC for me.

I do a bit of research on how to level them and in doing so, I get reminded of Sora's awesome Anti-Form. Granted it has some serious setbacks but I feel if you are skilled enough, its a frighteningly powerful form.
It is quite powerful and fast, but the drawbacks of taking twice the normal amount of damage, not being able to use Magic or Abilities, not being able to pick up HP orbs, and not being able to use Reaction Commands greatly outweigh the benefits.

Though there is one thing about Anti-Form that interests me. Anti-Form is the only way to give yourself the EXP 0 ability, (Unless you hack it, or play KH2FM) and one could use Anti-Form in forced battles to reduce the amount of exp received and play through the game at quite a low level. I guess this would be the only way to mimic KH2FM's Critical Mode challenge.