As far as R/Ms go, I'm a 70 R/M and I have never switched to my third class.
Is instances, I may not do as much damage as a R/S, but in siege I'm a force to behold.
My Booby Trap (AoE) hits most players in siege for 40k with 40k patk. When I have an attack sigil on, it does 100k on mobs (which is usually within 20k of what it does to player)
Paralyzing Trap (Single Target) is usually enough stun to kill a player with <100k HP, but for those pesky 120k+ HP healers who spam urgent on themselves, between Shadow Prison, Paralyzing Trap and Silence, I can keep them at bay for around 35 seconds (no casting for you

)
Poison Trap (AoE) is a bit weak IMO and upgrading the skill does little to help this. At 40k patk, my Poison Trap does 3.6k dmg/tick.
Poison Flames (Single Target) is a massive DoT... On players it does around 9-12k dmg/tick and on gates it does close to 20k.
With all of my DoTs, Bleed, Grievous Wounds, Poison Trap, and Poison Flames, I can take a gate down in about 100 seconds.
My favorite thing to do in siege is to camp a bridge and just place all my traps at the end on it... if my Booby Trap doesn't 1-shot my poor victim, I just go in with a few low blows and re-set my traps.
When large amounts of enemies are camped out at a tower, I like to Phantom Stab in, lay my Booby and Poison traps, then use Trap Master (Instantly places all your traps around you)... with all that, there are few who remain standing.