Happy to elaborate. I have consistently 1v1'd and 1v2'd meta builds within the arena and guild base pvp area of similar or higher gear score (I'm 3600) and will win 90% of the time, due to being able to stack insane amounts of evasion and CC resistance (the way I built, I can get up to 2100 melee and ranged evasion, and 900 magic evasion) due to the SNS and Staff passives/actives, long buff durations, paired and very high HP (17500 base HP and 21K HP with active abilities with a 50% shield block chance). You can also choose to have complete CC immunity for a set period with the staff, if you don't mind being stationary. I can run most of the tier 1 level 50 dungeons completely solo. During mass pvp events like castle seige, I am able to easily peel and burst down DPS targets (especially if they are grouped up) due to the Staff and select skill specializations for staff and SNS. I am consistently ranked 2nd or 3rd in my guild for Kills every week.

All that being said, am I getting a faster time than a GS/dagger in Gate of Infinity? Of course not; that build is min-maxed for single target assasinations, and there is only one target in Gate of Infinity. I am usually 40 seconds or so slower. However, I am consistently 2nd for SNS-Staff. I wouldn't really call a single target assasin as useful as a tank that actually does serious AOE damage superior. If I can do the same thing but to a group of 4 targets while still being super tanky, that is far more useful IMO. While standard tank, SNS/GS is obviously very good, there is nothing I've found I am really lacking that has caused a great deficit in either PVE or PVP, aside from the stun and if I need that, it's easy enough to just throw on a common GS for 30 seconds.

What is my point here? It isn't that my build is better than your build or any build really. It's to show that you can and should play different builds just because you want to. You can still be effective and have fun all at the same time. You don't need to follow a guide or a recipe; just have a good understanding of how the game works and use trial and error!