Main handonly/trap builds
People have made main-hand only weapon builds, as well as off-hand only weapon builds. The videos I saw for them were mostly humour-inspired dueling videos though, I don’t think anyone would ever seriously use one of them for general play.
The special #3 attacks are only really there because they had to have something in place in case an offhand wasn’t used. They’re generally just weaker versions of the other attacks though, so there’s never any real point in using them at all.
As for trap-based builds, I have made one for PvP. Shadow trap can be especially potent because it provides you with mesmer portal-like functionality, but with an extreme time limit. However, it does have distance and pathing limitations, which cause it to screw up sometimes.
However, I wouldn’t recommend using a trap build in PvP. The primary reason for this is because you’re giving up too many valuable things for something that is simply not as valuable. I feel trap builds (for thief at least) will only have any sort of potential in PvP if they add the runes of the trapper to the PvP gear selection. Having those would provide the thief with fairly reliable stealth mechanisms, making their traps much more effective, and making them less reliant on things like shadowstep to help them escape everything. Such builds are workable in WvW for roaming, although they’re still not as strong as more conventional builds overall.
For reference, the particular build I’ve used is: http://gw2skills.net/editor/?fZAQNAqaVlsMptplNxvJsPVBQBtVA4sDRwzkUEtycX3A-TpxCABAcIAU4EAQ1+DbVGgijAw0PAAA
Even for a trap build, I’m sure it’s poorly optimized, so a better player could likely easily come up with something better. I use hidden thief because I need that extra source of stealth since there’s no trapper runes. When I use this build in WvW, I usually drop hidden thief for sleight of hand or pain response, and I use a carrion/rabid/dire stat mix instead of full rabid. I also don’t recommend using corrosive traps, but I just wanted a build that used all the trap traits (the whole two of them).