Definitely useful skill. Some bosses and mobs have those "casting" attacks that are able to stun you, or wound you in some way, whatever, you know what I mean, so Guard is useful in those moments. It is not something that you are going to be spamming like crazy, but if you master it, it makes a difference. For example: As a Martial Artist, after you use a skill, there is a blank moment before the character starts auto-attacking, so it is a good thing to use Guard meanwhile.
But yeah, above all, Guard can be not used at all at the beginning. It must be used in some TMQ and situations if you don't want to die tho. I can remember wipes because people not using Guard.
About SP 3. No idea, never tryed it myself.
Definitely. UD2 is a good example on my part, because I always Guard while it charges. The Counter Attack is also a good skill to have as a KD effect.
Hmm...I could possibly test it once TLQs become playable. 2 SP to max the skill isn't a big deal after all.