Yes, all of the enhancements are cumulative, so they're all valuable to varying degrees. I favor the ones that give big damage or instant kills in group style (Precise Hit, Critical Hit), extra damage when used with Igni (Patinado), knockdowns, or bleeding.

Are you sure? I just downloaded the manual and on page 32 it says that style enhancements are not cumulative. However on page 49, it does say that attribute enhancements are cumulative.

Stuff like Cut the Jugular I or Crushing Blow I will only apply to the first attack, yes. Cut the Jugular II and Crushing Blow II will only apply to the second attack. However, you always have to do the first attack to be able to do the second, and so on, so it's not useless... you'll be using the first attack more than any of the others. You're also going to have enough copper points to choke a goat fairly quickly, so it's not really a big deal.

Other, non-sword-style bonuses are almost always cumulative, though. For instance, all five intelligences will get you +100% sign intensity (10+10+20+20+40). Gale (for aard) is cumulative with Gust, Thunder, and the base bonuses. However, each level of a magic skill completely replaces the previous one, so for level 3 aard, you'll get +50% (plus bonuses) intensity, not +75.

In general, though, sword-style bonuses only apply to the attack on the same line, while all other bonuses apply universally and cumulatively.