Stealth is incredibly useful as Liquid Wolf said. If you go with Nightblade, consider the Shadow skill tree. Shadow Cloak (invisibility) counts as stealth so any abilities that are affected by stealth will be affected by Shadow Cloak. For example, Veiled Strike "sets enemy off balance and stuns them for 4 seconds" if stealthed, and that applies if you are using Shadow Cloak as well.
If you're fighting an enemy up close you can't go back into stealth because you'll be detected, but Shadow Cloak allows you to do that. I use Surprise Attack (morphed from Veiled Strike) while invisible because on top of the stun, it reduces enemy armor by 40% for 12 seconds, so you can get more DPS in there. I'm not even playing melee, but if an enemy gets too close I can pop a Surprise Attack to weaken it and create distance. It's versatile.
I'm only level 10 so I haven't explored much beyond that. But a melee DPS Nightblade will naturally want to go with Assassination, and some Shadow would be really beneficial.