From my past expierence invisibility was allways a extremly strong ability especially in PvP. The beauty of this skill is that it can be used in so many different ways and that it can't really be counteredYou say that you should use more specific abilities instead of invisibility, but I haven't seen anything which comes even close to invisibility.If you have the ability in mind which reflects projectiles than you should keep in mind that most magic attacks doesn't count as projectiles that AoE doesn't count and also not melee attacks. If I would play a Nightblade I would use invisibilty for sure, but that's just me.